I've found weak flippers on the new Stern spike machines to be caused by:

1. Mis-adjusted EOS switches
2. Coil stops that have started to break down. The rivets that hold them in can break away slightly, which causes a little wiggle in the back of the coil that can have the plunger bind a little bit. If you see gold dust in the bottom of your cabinet, this is probably the case. Since it's HUO I'd be surprised if this were the case but check anyway.
3. Bad Node board. There are two node boards on Spike systems under the playfield: Node8 and Node9. I believe it's Node8 toward the front of the playfield that drives the flippers. If it's intermittent swap these two boards (be sure to change the jumpers on them) and see if the problem goes away. If so you'll need a Node board replacement for the bad one