what could be the problem on my Transmission?

Question

I have recently changed the Input and the Output sensors on the transmission, my problem  is after it shifts into first gears it never switches to the second or third, but that usually happens when the check engine light goes on, but sometimes it takes a long time ( High RPM) for the first gear to shift. Also when I erased the Check engine light code (comes on once or twice  every 2 weeks or so than it disappears sometimes) the trans starts to shift like normal.My question would be what would be the problem since I changed both of the input and output sensor and also transmission valve body? Could it be  low on trans fluid and a potential clogged filter ?