Well, my guess is that it's not ready. If it's healthy, then it will lay an ooth sooner or later.
Another thing might be that it can't find a suitable surface to lay an ooth. I don't raise mantis in these deli cups. In my enclosures, where there are a ton of different surfaces and materials to choose from, they (different type of mantis, but might provide similar insight) choose the most unlikely spots and are very picky as to where to lay the ooth.