I bought a glacier bay 1000 055 745. It's supposedly rated at 2.5 gallons per minute. It's not much more than a dribble though, making a shower much longer to rinse off soap.

Is there a way to open up the water flow?

Remove the shower head and tell us if you have a really good stream of water, I think the head is clogged with debris. Seen the CLR commercial?