Lvl 4 trainer is not the problem. If you're training bigs, I assume you're training Carpieiro and Senneville, the 20 and 22 year olds. That makes training a bit slower, but you should still get pops. 2 position training is indeed slower than 1 position training, but they should still pop. Are they getting enough minutes at the required positions? (I guess so, but you can check). What are your expectations? Maybe you should look up the training speed topic (
(78242.1) and see what you can expect.