727 slow to move (1st or reverse) after sitting a couple days?

[70 Challenger Conv.]

I have problem with my 727 not moving like it is slipping only after it sits a few days in reverse or 1st gear. I can give it a little gas and it will slow start moving and then seems fine afterwards. The fluid doesn't smell burnt and is full checked in park as stated on the dipstick. Any ideas where I should start?

[tactransman]

If it is fine after the first engagement of the day,it is converter drain back (drain down) it is normal. Try starting it in neutral next time.