One other possibility would be the prop size and pitch not being the most efficient.

More info on the GWS 100 is here:

http://www.aeromicro.com/Catalog/gws...2s__803598.htm

Note that the 9047 or 9070 do the best job. Also are you running Nicads or NiMH? The 6 cell pack is OK on Nicads, but a little weak on NiMh. I run 8 cell NiMh on my 100s motors. They may shorten the motor life a little bit, but much more zippy.