The fine porosity frit may have been such a high resistance that the compliance voltage of your potentiostat was reached! For most commercial designs this should have shown an OVERLOAD condition since the potentiostat was no longer holding the desired potential. The actual potential would have been the compliance of your potentiostat minus the iR drop across the frit minus the voltage required at the counter electrode to sustain the current on that electrode.It will change as the current changes through the electrolysis.
As you didn't give any of the particulars (potentiostat, compliance, or current) it's hard to back calculate, but the efffects should be similar to those you reported.
In the really old days we used to add some hydroxylamine to the counter electrode compartment to act as a depolarizer for the counter electrode and reduce the required voltage at that electrode. You also didn't say anything about the concentrations you were using or the supportting electrolyte. Try decreasing the concentration of your analyte and increasing the supporting electrolyte concentration.
Some words about compliance voltage are at
Drop me a note if you want to discuss this further.