Low output on a single string is normally due to one of the following:

1) String is noticeably higher from the pickup than other strings. Physical adjustment needed to bring the string down tpo the same relative height as the other strings. An obvious cause of the problem, and is going to be fairly rare.

2) Pickup pole piece heights need adjusting. You can try raising the pole piece heights for the G and E strings until the volume output is the same as for other strings. Which again is unlikely as the main cause as most P90 sets work well without any adjustment, which makes it more likely that it's a ....

3) Duff string. Try another string. If they are the Chinese set that came with the kit, I'd swap the whole set out right now.

4) Otherwise there is something stopping the string vibrating properly. You'd normally hear buzzing associated with this if the string was too low, and it's also likely that the volume would vary depending where you played on the fretboard.

But my first choice (without being there) is a dead string; at least for the G string. The top E could be a height adjustment thing if it's less severe.