Transmission won't find a gear

Putting it in park with the wheels turning will make me cringe, but it won't hurt the trans any.
They are made so that it will give until the wheels go slow enough for the cog to engage.
Linkage (Make sure it is engaging into each detent on the trans correctly)
Filter (Be sure it's the right one for the year of the trans and installed correctly)
Converter (not put back in correctly and damaged the pump)

This is what I would suspect, and the order I would go after it. (since it worked before)
Unless there is something that hasn't been mentioned.