It could be that the cap itself is leaky. If it's an electrolytic cap it might have a fairly high leakage current. You could try with another type, like ceramic of foil.
Or the probe itself might be dragging it down, but I would expect it to have ~10 Meg input impedance, usually. For multimeter anyway, oscilloscope with a 1x probe would be 1 Meg which would drag it down to about half Vdd.