Hi Cobbler, You could have an air lock in the cartridge.
I use the Ink-Tec refills from click here and with them you get a cartridge holder, a rubber attachment and a special syringe so that you can actually withdraw ink through the print head and thus make sure that the ink is flowing correctly.
I suppose it could also be caused by leaving the cartridge empty for a while before refilling thus giving it a chance to dry out slightly.
Try standing the cartridge on a wet tissue for quite a while to try and get it flowing properly.
You haven't sealed the refill hole have you???