What I'm finding on most TNAs that I've worked on is the flipper bats are adjusted too tight to the playfield. They will work fine at full power when cold, but as the assembly warms up and the parts expand, the flipper bats begin to bind slightly resulting in a weaker flip.
Properly adjust the flipper so there's enough of an up/down gap in relation to the playfield to allow for the parts to expand when they get warmer.
I've also found on most new manufacturer's games these days that the flipper cabinet switch isn't adjusted properly. Getting a good, solid closing and swiping action on the contacts is very important. Weak flippers seem to occur if not adjusted properly.
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Rob Anthony
Pinball Classics
http://LockWhenLit.com
Quality Board Work - In Home Service
borygard at gmail dot com