If the bow is across the grain, that is called cupping. A bow is along the length.
This is a common problem with panels. Relates to factors such as stability (dryness) of wood, accuracy of jointing, how panel was stored, and the wood itself.
Not know how the wood was prepped, but Iíll offer some basics:
If lumber isnít stabile or acclimated, it will continue to move after the glue up.
A good practice in jointing by machine alternate faces to cancel any minor error off 90į.
Once the panel is glued up, it has to be stored or ďheldĒ in stickers or cauls. Can also put them in plastic bags. On small panels I put them in stickers with either a weight on top or a strap clamp around the stack. On large panels I generally use cauls.
Flat sawn wood is the least stable, but usually the most desirable for panels, as they have the nicest grain pattern, so itís very important to hold the panels and let them stabilize prior to assembly.
The best way IMO to fix a cupped panel is to cut it apart and start over.
Which reminds me, keep your panels oversized until right before assembly, just in case ;-)