That's too low. I had a 75 and it cut clean holes, I did not have a chrony at the time. But it should at least match it's less expensenive sibbling the HW40 which I have Chronied, one was putting out 390 fps average and the other 360 fps average with 7gn Hobby & Geco.

First thing to check is the breach seal/barrel, sometimes one or both need adjustment to obtain the optimum seal. After that consider the main pistol seal. Have you lubricated the piston seal since you bought it (check out the HW75 manual on line).