Sorry if this is blindingly obvious, whilst I'm familiar with centre fire and rimfire rifles,I know little about airguns.
I have a HW77 which shoots groups in vertical strings.Ie 1/2 " wide by 3" or more high at 35 yards, I'm holding the gun firmly.It's not the scope or mounts ,I swopped them.What is likely to be the problem?

Thanks for your help.