Transmission doesnt engage at low rpm, any advice will help Options

[rapid36]

Hello everyone my celica has been running great for years after my rebuild and i always had a running rich check engine light. I recently now am having troubles sometimes but NOT ALWAYS engaging into a gear from either a dead stop and trying to go again or shifting into a new gear. Seems like when im going and driving it doesnt have any issues going into any gears but from a dead stop it sometimes wont get into the gear unless I rev up to about 1.5k-2k super embarassing at stop lights as you can imagine. Any advice would be appreciated. Let me know what you guys think that I can start checking.

Things I have checked.
Transmission fluid is full and havent had to add any.
No new check engine lights or codes.

Thanks for your time!