Hi Amy I've never made this but am wondering if you put boiling water in the jug to start with (cold water would lengthen the cooking time) and if the frittata was in the upper or lower tray of the varoma. The bottom tray is deeper so would take longer to cook.
Amy, after the first 15 minutes, did you reset the varoma temperature or did you just reset the time? I was going to suggest turning the speed up higher but I see you had already done that. If you reset the temp every time you reset the time, topped up with boiling water when it needed it then I just can't understand why it wasn't set. Did you stick with the quantities as set out in the original recipe or do you think perhaps yours was much thicker. I haven't done this for a while so I can't remember what mine was like.
Amy I make omeletes in the varoma regularly. They cook quickly,I always use boiling water, thinking maybe your fritata was very tall? Try making an omelette next time & see how you go.H