Make sure the valve core is tight in the stem. I just had this happen on a wheel, the valve core got slightly loose, and it would not hold air.
But wait, I just noticed OP says he can't seat the pump on the stem, that indicates he might be using the Schrader setting on the pump, as noted above.
Last edited by AndrewJB; 08-12-20 at 09:14 AM.