What is wrong with my transmission

I will be driving up and down the road and notice that the engine will rev and sometimes not shift in gear. It will shift into first fine at 10 miles per hour and then around 22-25 the engine will rev and wont go into gear unless you step on the gas more and give it more power. I am wondering if maybe the torque converter is causing problems or if the torque converter clutch solenoid is going wrong or if its ineveitable and the tranny is starting to slip and the clutch band that lets it shift into the 2nd gear cluster is no longer strong enough to engage. Any help would be appreciated.