I grow in containers too, and I agree with Zipcode - fertilizer makes all the difference. I'm still looking for the perfect stuff to do it all organically. Potassium is so important for sweetness and fruit quality. Nitrogen I think plays more into the overall yield, and it's the most soluble of nutrients so it runs off and has to be replaced in containers... I recently scored a huge bottle of Alaska fish ferts, so that's going to be my go to fert for 'fruiting season' ie once they start to ripen the first ones, they need to be fed on a regular basis. And pH drift definitely can be an issue, and affect the uptake of phosphorus and potassium. Also the weather! can affect potassium uptake.
So if you had good seasons and then a bad one, could be a difference in the weather was most unhelpful of all.
Too much sun or too much shade - both can be a problem.
Maybe you should post a list of your 'too many seeds'
from the swap, and folks will tell you which ones we have grown and done well in containers. Being in LA, I would think some of the great tomatoes by Fred Hempel and by Wild Boar Farms would do really well for you.