I think BV is right - your filter material is probably too dense - are you
using sponge? - you need to maybe get some filter material with more
airspace (i.e. not as fine) so it won't clog up as quickly
Gale :~)
I built a filter with
3 plastic planting pots. In-between the 2 outer pots is 1/2 gravel.
In-between the 2 inner pots is small pebble sized lava rocks I
purchased from the local pond store. Inside the inner most pot, where
the pump is, I lined with washable filter material. The filter sits in
the lower pond. All the pots have many holes drilled in them and it is
cover on the top with the saucers from the pots with a hole drilled in
them for the bell fountain. It works great and the water is clear and
doesn't have an over abondence of blanket weed. Here is my problem.
The pressure goes down once a week and I have to take the filter
material out of the center and rinse it off.
snip

I use purely a VF, and have as a rocky stream to give me some more real
estate for bio bugs, so I am not speaking from experience, BUT, it sounds
like what you are describing is a common mechanical filter issue. You are
filtering particles out of the water by catching them in a your filter
matter. Once that matter gets plugged up, the water can't flow as fast,
reducing your pressure. I think anyone with a basic mechanical filter
system
will tell you this is normal. I am sure the big dudes with bead filters
and
the like don't suffer from this, but it sounds exactly like your problem.

BV.