What could be causing my automatic transmission not to shift?

What is your question?

Sometimes it will shift on its own but most of the time it doesn't want to. I can get it to shift most times when I let off the throttle, put the gear shift in neutral, then back in to drive, and press on the throttle again. The fluid level is OK, it hasn't thrown any codes, and the transmission doesn't slip any. Do you have any ideas on what could be causing it?