T-5 fluid/problem

[Just me]

Hello. I'm having a hard time with my t-5. I replaced the fluid with ATF DextronIII a month ago. I put the right amount. I don't actually remember what I put in for fluid the last time I did this. My problem is that the tranny shift fine and smooth, but after about 20 minutes of driving, I can't get it into 1st or reverse without fiddling and fighting the shifter. I replaced the clutc/pressure plate, rear main seal, bearing, etc. recently so all works well with the assembly but the t-5 has had this annoying problem before and after the clutch build. Any ideas? Thanks

Just me