Have installed 12x24 inch porcelain tiles in our kitchen on top of old tiles. Used a premixed tile adhesive (which a friend used in his kitchen on top of old tile with no problem). After 5 days it has not dried fully (only dried in a few spots). Will it dry eventually or do I have to start over?

The problem is the premixed adhesive, Its great for 4x4 or 6x6. But for the bigger tiles it will not dry, It isn't getting enough air to dry,

Always use thin set mortar for floor tile of any size, use premixed for backsplashes that have 4x4 and 6x6 tiles.

you tile will dry in time but it may be a while and I can't say it want pop up later,
if a tile is setting and you move it, it will break the bond, in doing so it will weaking the final bond it has once dried,

if there isn't a lot of tile id pull them up and do it right, the motar that is on the floor right now will come up easier now then a month from now.

good luck