Note that, because of the dish of the rear wheel, the spokes on one side of the wheel will always feel looser than the other side, or the front wheel, which has no dish.

I'm a big guy too and I've had the same problem with a couple rear wheels. Both times I had the wheel trued a couple times, messed with it myself a few times, and finally had the wheels replaced or rebuilt by my trusted LBS mechanic. The replaced and rebuilt wheels were both trouble free after that.

If the bike is still under warrantee you can get the shop you got it from to mess with it for free. But, if they've already given it a try or two, and if you want to get a trouble free wheel, get recommendations for a really good wheel builder in your area and buck up to get it done once, properly. A good mech will tell you honestly whether the hub and or rim can be re-used or whether to just get a whole new wheel.