First thing I'd do is check it with marksman pellets, to confirm whether it's a true .25 or a BSA .243.
The .243 barrel will rob ME from .25 pellets due to the tight fit.
If post 2012 it should be true .25 but always worth a check, & marksman are a "small" .25 pellet at around 6.2mm.
Then it's pellet weight, at sub 12 power launching a heavy pellet is a struggle for the available energy, having done a lot of past testing 24gn(ish) is the tipping point,
go over 24gn & muzzle energy falls off a cliff because the available sub 12 energy, just can't accelerate the weight enough inside the bore, so again Marksman at 19gn ish, & FTT at 20.06gn.
JSB at 25.4gn will be starting to drop the muzzle energy, and frankly don't bother with anything heavier than that, 28gn will be 1.5ftlb down, 30gn probably 2+ftlb down
Of course if yours is a DIY conversion, it may just be poorly set up & going back to a spring may be the answer.