Could it be that the left flipper isn't wired correctly? There are two coils (high power and low power) wound together. Usually the closed EOS switch bypasses current around the low power coil until the very end of the stroke. When the EOS switch opens current must go through both coils effectively making the whole thing a low power coil. If it's not wired properly, you could always be using the low power coil and switching the high power coil in and out with the EOS switch.

There's more detail here if you're interested:
www.funwithpinball.com/learn/flippers-coils-and-power