Hey Drew,
There are a few things here you can try here but it depends on what your wants are.
Are you just trying to accomplish more revs or return to your previous rev rate? It's possible your wrist has gotten weaker over time (not sure of your age) and you're not snapping your wrist like you used to which is also causing the lower rev rate. This can also come naturally from bowling on drier conditions over time as we (bowlers) will tend to adapt to our environment.
You can also try a lower rg piece which will replace some of those revs but if it's a loss in reaction you're encountering, then maybe going to a stronger piece will help you in that area.
Checking our spans and hole sizes periodically to fit properly is never a bad thing. As we get older, we'll lose some elasticity in our skin, flexibility of our joints and/or loss of cartilage which can change our physical attributes. The necessity of shortening your span may be needed if anything mentioned applies.
Since you've been throwing without grips for so long and something has changed, I'd lean more towards the fact of losing some finger/joint or wrist strength which you had more of 5 years ago. Hope this helps some...
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Rick Leong - Ten Pins Pro Shop
Tag Team Coaching - Co-Founder
"El" Presidente of the Legion