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!

[Box]

How many miles has it been since the rebuild? Supposed to change the fluid and filter every 25,000 miles or so.

[rapid36]

How many miles has it been since the rebuild? Supposed to change the fluid and filter every 25,000 miles or so.

Its been 15k miles since the rebuild. Its probably close to that time then . Could give that a try thank you! Also where is the filter? Are you just referring to the magnets in the transmission oil pan?