The maximum cog size is limited by the distance between the upper pivot and the top of the upper pulley.

My guess is that the upper pulley does not have enough clearance to shift onto the larger cogs.If the derailleur has a "B" adjusting screw (the one on the rear of the upper pivot), try screwing that in a few turns. That screw presses against the dropout adapter. Tightening the "B" screw forces the derailleur to rotate backwards which effectively increases the distance between the upper pulley and the pivot.

I bet that derailleur will handle your 25T cog if you're running friction shifters.