Originally posted by Rage.Of.OlympusMentioned this earlier, but when Prime first appeared in this series he was being held under a magical red sun that kept him depowered. Once free, the implication is that he went to earth very soon afterward to fight Shazam. Aside from that, the fight itself also took place at night, so there was no solar energy for him to absorb(which, as we saw during SCW, can certainly hinder Prime if he's not already at full power.)
Maybe. It wasn't mentioned in this issue. Maybe I missed it?
And on the SCW note: remember that Prime had been unleashed for quite a while without ever reaching full power. I mean, they broke him out of his red sun prison and gave him his solar-collecting suit right away(in the very first issue of the event)... But even by the time he invaded earth with the army(which occurred near the very end of the event), he still wasn't at full power.
So it would be perfectly in-line with Prime's existing history if he wasn't at full power when he fought Shazam/BA.