Two possiblities.

- You have a brushed version Stryker it seems, so it is possible that the brushes are worn down.

You could try putting in a new motor.

- Don't rule out a problem with the battery packs. The only way to really tell is by using a Watt meter.

It is possible that the replacement pack you received has been sitting in storage for a long time, so the battery is not properly "conditioned" or is no longer good.

You should run it down, then overcharge it by about 10%-20% ( timewise ) before trying it again to help condition it.