Water filters always reduce pressure to some extent. Next time you may want to try the C1-20 filter, which has less of a pressure drop.
http://www.waterfilters.net/Pentek/wholehouse/Carbon_Filters/C1-20.htm
Or, you can get a bigger GAC filter, which has much less pressure drop too:
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/GAC-20BB.htm
This would require a bigger housing of course.
Thank you,
WaterFilters.NET