The fill to the blue barrels should be from the top, not the bottom. Something about gravity.

I have seen it set up both ways and I preferred on the bottom. Just need to know what adjustment to make.

Take one or all the lids off the blue barrels and you will find out immediately if air vent holes are what you need. Loose lids might be enough. Water certainly is not going to flow into an air tight container full of air. And the barrels should fill equally not one at a time.

Can you show the piping from the side? You say "in series" but it looks like it's more parallel, with pipe going down bottom sides of tanks and feeding individually. I don't see why the first would fill if all are identical.

Are those blue barrels upside down? If so, there may be no vent opening available on their top side now. Simply drilling an 1/8" hole in the tops of the blue barrels will be sufficient to vent their trapped air and let them fill.

Formulaross20, yes, it's upside down and thats what I thought as well, I need to drill vent hole on top of each blue barrel.

Seabornman, there's no pipe along the side. Since I flipped the barrel upside down, I put one short pipe, L connector at one of the bung caps and link all barrels with T connector.

If the blue barrels are upside down isn't that going to increase the chance of the whole system leaking from possible lids or existing top vent holes? It is obviously air tight now since the black barrel is filling. And I know nothing about these blue barrels.

Putting the connecting pipe on top is the difference between the barrels filling one at a time or equally I suppose.