I have never ran a NFT system myself, so I don't have direct experience with water flow. But not knowing any details of your setup makes it hard to know what is going on.

1. I'm not sure why you can not increase the angle of the slope anymore?

2. Also I have no idea what that angle is?

3. I don't know what type of tubing you are using (round, flat, etc.)?

4. I don't know if there is a diffidence in water height from one end of the tube to the other. It sounds like from the description the water level is the same at both ends of the tube? And that would defiantly require knowing how you have it setup.

5. I don't know how old the plants are, and/or if any other conditions have changed. I'm also not sure if you have both seedlings and mature plants in the same tube?

Lastly water seeks it's own level, any fluid does. So If water level is higher on the sides in a flat tube (even round), the water flow may be too fast. Like a fast moving stream or river with obstetrical in the center of it. If the water flow is fast, then it wont have time to settle in all of the gully's (low points), as it goes around the obstacles (in this case root mass) making the water level higher at the sides. Do you have a way to inspect water levels thought the tube, or just at the end.

I'm thinking it's related to the water flow (speed), and this can be altered by the pump flow or the angle of the tube, and/or even adjusting both to get even (or at least better) water levels thought the tube.

Bottom line, even though you should be able to adjust the water height, unless some plants are growing faster/slower than others (in the same tube) I don't think there is much to really worry about. Sounds like the roots look healthy.

It could be environmental problems.(air temperature, water temperature, lack of light, etc)
It could be the age of the plants. how old are they? what is their life cycle
you stated you couldn't increase the slope previously because you seedling roots wouldn't be wetted. could you increase the slow now, as you don't have seedling to worry about any longer?

I'm using rectangular rainwater downspouts (flat bottom). I can increase the slope now, since there are no seedlings. Water depth tends to increase on the upstream side of thick root masses. Not particularly at either ends of the tube.

I guess I should clarify this part: I set the slope before transplanting the seedlings, when the tubes were empty. Knowing that stagnation could be a problem once the roots grow bigger, I set it as steep as possible. When you set them too steep, the water is not distributed well on the bottom of the channel, some parts are left dry.

The tubes are 4 meters long and the flow rate is about 1 liters per min.

What I really want to know is, Is it possible that the roots have enough oxygen to live yet not enough for the plant to grow properly? In several resources that I read, It is specifically stated that the film thickness should not be over 1-2 mm. or the lack of oxygen will kill the roots. Now I have up to 8 mm. thickness, maybe a centimeter. At some places the roots are literally swimming in it. Yet they are white.

I use rectangular downspouts as well (ridged not flat though) that are 5 feet long (~2.6m)and haven't had plants stagnate from it. as long as your water in the rez is well aerated, It should have ample oxygen in the shallow pools for no longer than the run is.
I started my slope at 2.5%, then raised it to 5.8% when the plants got larger.
It's probably not ideal NFT, but your plants should still be growing.

Hi aegean, your problem believe it or not its more than likely oxygen depletion. If all of your plants at the top of your setup are doing better than the rest, and they succeedingly get worse down the line. . Its probably O2 depletion. One would think that even an areorator would do the trick but more often than not it simply isn't enough. Try adding another to your resivoir pool or making your channels wider to allow your solution to flow around the rootmass. The more each rootmass ball slows down the flow of the solution the worse it will be. . . Because those small pools of water or solution are literally gettin the life sucked out of them completely depleting the remainder of the O2 in it. Run some tests on the oxygen on the water at the end of your NFT setup before it dumps back into the resivoir on your current setup and see what you come up with

Hi aegean, your problem believe it or not its more than likely oxygen depletion. If all of your plants at the top of your setup are doing better than the rest, and they succeedingly get worse down the line. . Its probably O2 depletion. One would think that even an areorator would do the trick but more often than not it simply isn't enough. Try adding another to your resivoir pool or making your channels wider to allow your solution to flow around the rootmass. The more each rootmass ball slows down the flow of the solution the worse it will be. . . Because those small pools of water or solution are literally gettin the life sucked out of them completely depleting the remainder of the O2 in it. Run some tests on the oxygen on the water at the end of your NFT setup before it dumps back into the resivoir on your current setup and see what you come up with