You might not be looking in the right place: you said the flipper assembly is clean, but you’ve been focusing on the flipper drive power. Have you checked the flipper bushing? Sometimes they can be subtly cracked, not visible without removal and inspection. When you try firmly wiggling/tilting the flipper from above the playfield, does it have any more looseness or range of motion than the other flipper? It would explain different apparent strength in different shot directions...