WHAT CAN I EXPECT??

I used to own a .177 HW97 and it was very effective on tree rats - tuned to 13.5 FPE and slinging 10.5gr brown box CPs.
My feeder distance was 22 yards.
One thing I would suggest to you is hit the heart, not the 'gut'.
Same with 'head' shots ----> Hit the brain not the 'head'.
Need to be very precise with .177 for 1-shot kills.
 
When I am hunting, I've got at least one dog with me. If a squirrel or rabbit takes a bad hit, the dog will find it and finish it. If it gets back up a tree, the dogs help me find it again. I hunt small game with .177 caliber rifles, heavy ammo and aperture sights. I can usually stalk for a head or chest shot and I can pass on anything less. Still and all, I like having a little bit of insurance for those situations and the dogs are thrilled no matter how a hunt turns out.
 
Looks like the HW97 is under 15 fpe in 177. I used a Prod initially for the first 4 squirrels I shot with an airgun and lost 2. It was about the same fpe in 22. I increased the fpe to about 18 and have had considerably better experience since. The only one I've lost of the next 14 flipped itself in the hole below a large oak that had root rot. It was brain shot.

I think your issues are really two fold. Your power is a bit on the low side and your placement is not as good as it could be. Either by itself wouldn't be too bad but together it results in what I call "runners". The squirrels I've shot in the chest with my 177 (about 19 fpe) have run 5 or 6 feet and expired. Except for one I shot in the behind. I never found that one. Bad shot by me. Not the gun or calibers fault. The other 23 I've shot with my 177 were all recovered. My dog found the remains of the one I shot in the behind later.

If you hit the body solidly I think the squirrel died. If you only hit a leg or the edge of the body it may have recovered. But it might have gotten an infection and died.
 
I exclusively take headshots on anything larger than a rat. My HW30 does no problem head shots out to 20 yards. All my PCPS are capable for the max distance in my yard.
I’m in a suburban neighborhood, so my rules is to drop anything I shoot on my property. I will only take shots that lend to that philosophy.
In my opinion find a rifle that will provide that, or get your skills capable if your rifle has the capability. I’m not trying to question your abilities, there is always room for everyone of us to be better.
I use a kill zone for all my pesting, and I practice that distance with the same support probably 1000+/1 the kills I take.
 
Hi Airgunners !

I use my Beeman R9 .177 with JSB’s 16.2 gr to get rid of iguanas , our game and main sport .

I get them at 30 yards no more . Also brake two springs , maybe is because the heavy weight pellets !? 🤔
That’s what I have discovered from my research that heavier pellets are hard on the spring. Have you tried aftermarket springs?