No hot water from Valve Body WITH cartridge removed

Recently bought a house, built in 2006, and I've found the MBR shower doesn't have any hot water, so I got a replacement cartridge (a Gerber 95-154, see photo) and figured that would fix the problem, but it didn't. So I removed the escutcheon cover plate and happily found it had hot/cold shut off valves in each arm which allowed me to remove the cartridge and check the hot water side (second photo). In the second photo you can see the hot water valve is fully on and cold water valve is fully off, and no hot water at all coming from valve body face. BTW, I do get very nice hot water at the garden tub located right near the shower, so I know we have a working hot water heater. So my question is why, does this indicate a bad valve body? Is there anything serviceable in the valve body, I see a plastic white insert but don't know if it's removable or not, or if they get clogged and need to be cleaned. I suppose the other possibility is the hot water shut off valve is stuck closed, and I see a large nut around the screw adjustment, could I remove the large nut and inspect the valve (after shutting off the main of course). Or are there other issues that might be happening here? Appreciate any comments. Attachment 49797 (http://www.doityourself.com/forum/attachments/plumbing-piping/49797-no-hot-water-valve-body-cartridge-removed-balance-cartridge.jpg)Attachment 49798 (http://www.doityourself.com/forum/attachments/plumbing-piping/49798-no-hot-water-valve-body-cartridge-removed-valve-body-only.jpg)