In this case, cycling the pack won't help enough to make a difference, UNLESS the battery wasn't charged in the first place.

What are you using to charge the pack?
How long does it take to charge?
What charge rate are you using?

You should be using a peak-detect charger capable of charging 8-cell packs. The charge rate should be no more than 2.1 Amps, and the charge should take about an hour at 2.1 Amps. Divide the capacity of the pack by the charge rate to get an approximate charge time.

My suspicion is that these "2100mAh" cells are high-capacity AA cells from Wal-Mart or Radio Shack. These cells are not designed for high-current applications like R/C airplanes; they're designed for digital cameras and toys. You'll be lucky to get 5 Amps out of them, let alone 20 Amps. It's best to stay away from AA cells in general for electric flight.