That site shouldn't be a problem.

I'm no expert, but the only thing I've ever heard of that opens groups up beyond a specific range, is excess drag. Do you have big vanes on your arrows?

The other thing may be a form problem. Grip torque or collapsing on the shot. When you are close, you subconsciously know that you won't be able to see the arrow fly, but at longer distances you may be peeking?

Good luck,
Allen