lilmssass,
Do you have any way of checking the nutrient strength?

We had problems with our soil tomatos this year with leaf curl which I understand is a viral disease that tends to affect plants that experienced too much cool and damp when they were starting out.

Your problem sounds a bit different which is why I wondered if you have a way to check your nutrient strength. A common mistake is to let the nutrient get too strong and the plants can get burned. Tomatos generally like a pretty strong nutrient but anything will eventually say too much. You are not likely to experience a nutrient deficiency using a pre-mixed nutrient so long as you are diluting it at the correct ratios.

You say you keep a close eye on the water level so it doesn't drop, when you add to it to top it up, are you adding plain water or more mixed nutrient? Most of the water loss is going to be just water loss from evaporation and transpiration which will have the effect of making what is left in the resivuar stronger so most of the time you only need to top it up with water. This is much easier to see if you have a meter that will show you the nutrient strength.

You could try adding more water to weaken the nutrient strength and see if the plants respond positively.

As for when to switch to bloom, hopefully some one with more experience in tomatos will chime in. I've seen plenty of little tomato seedlings already with blooms when you buy them at the store. If no one else has advice, you could probably choose an arbirtary method of deciding when to switch. Something like a certain size or amount of time from when you started it. Light levels can affect blooming plants, if your previous attempt wasn't getting enough sun or if they were getting too much nitrogen those things can retard blooming in some types of plants.

Good luck

>>The 1st attempt, all the tomato plants did were grow wild, they never did fruit.

Depends what type of tomatoe you are growing. Some can be fussy to fruit like brandywine.
For tomatoes I only use flowering nutrient from seedlings all the way to mature plants. I give a weak veg solution about once a month.
The easiest tomatoes are cherry tomatoes.Sungold is great.