Charging a capacitor with MOSFETs
...probably because Vdischarge is not low, e.g. just floating!?

BTW: You don't have any current limitation for discharging the capacitor.

EDIT:
I guess actually your problem is that C1 is much to small for what you expect.
47pF is a incredible small capacitance and it discharges even with a 10MOhm resistor within some 1/10s of a Millisecond (47pF * 10M&Omega; = 0.47ms).
I don't know how you measure the voltage across C1 but I guess the instument (and the PCB or bread board) has a resistance that is in that order or even much less.