First...I'm in shock that a bow rated up to 337fps is only getting you to 260. I'm shooting a 4yr old Diamond Liberty and shooting 270 with that.
I'm no expert, but here's a few possibilities.
1. Are you shooting the bow maxed out, or turned down? You are way better off in all aspects shooting a 60lb turned tight than a 70lb turned down.
2. Are you shooting the right arrow for the bow?
2. Are you shooting a flat black arrow or a camo one? Camo arrows, the camo is usually an additional layer making your arrow much heavier and thus slower. Shoot the same rated arrow in flat black and it'll be much better.
3. Are you shooting with wraps by the fletchings? Again, the wraps are nice for vision, but they weigh more too.
4. Do you have additional silencers on your strings? An odd peep? Everything you add to a string or arm of the bow slows it down. Might be quieter, but it'll be slower too.
Lots of factors can change speeds in a hurry. Even feathers vs vanes can make a 3-4fps change in speed.