Did you actually measure the draw length? He shouldn't have to "swear to it" You were in the shop, why didn't they just measure it for you? I refuse to not know what my draw length is within a quarter to half an inch.
How long are your arrows? They should be close to what your draw length is, unless you shoot an overdraw of some type. If you are supposed to have a 30 inch draw length, have 29 inch arrows and they hang past the front of your riser at full draw, then your draw is most likely short.
If your draw length is wrong, how do you know you have the correct arrows for your set up? The correct place to measure you arrow at fulll draw is right above the deepest part of your grip. Normally one of the mounting holes for the rest will line up with this point. Mark that, and add 1.75 inches to it, that is your AMO draw length. Your correct arrow length should be about an inch past where you marked the arrow, for a conventional rest set up. It doesn't have to be perfect though. Like say you have a 30 inch draw length, you could cut your arrow between 30 and 29 or so inches and still be good. The spine charts are guidlines and starting points, not hard etched in stone science. Every bow is different. The important thing is to not get too weak of an arrow, that will make broad head tuning difficult.
If you shoot some sort of a drop a way or other type rest with some sort of overdraw to it you could shoot shorter arrows if you want. Just make sure you account for it in the spine chart. The charts get your draw length from your arrow length. Give the wrong arrow length and it will assume your draw length is something other than it really is.
Here are two links for you to check out. Must reads for the inquisitive archer.
http://www.huntersfriend.com/bowselection.htm
http://www.eastonarchery.com/downloads/
Check out the 2004 shaft selector software, and the Tuning guide. The shaft selector has the tuning guide in it pretty much. The other is a PDF file. If you don't have acrobat there is a link at the bottom of the page where you can download it.
Where did you buy the bow? Most mathews dealers would have made sure the draw length was correct to begin with. And if not should swap out cams for you to get it right. They may even do it if you bought it used, as long as you are trading the cam, not purchasing a sperate one. It shouldn't be hard to tell if your harness or string is the wrong size. I think the demensions of the bow would be off. Brace height, axle to axle that sort of stuff. Not 100 percent sure on that though. I'm not a dealer or anything. You may need to find a better shop.
I'm sure some mathews nut will answer here though.
Good luck,
Paul