Switch your game up a bit- jiu jitsu isnt about what you want- its about what your opponent gives you.
In the positions you're describing, its difficult to land submissions because your opponent is doing the correct thing (keeping his posture).
Consider trying for sweeps/reversals in these positions. When people are upright, they're often difficult to submit, but give you a better chance of getting under them (say, with a butterfly, half, or X-guard).
It becomes a game of chess- if you can keep attacking, you eventually have the chance to catch your opponent if he doesnt keep up with your attacks- by defending your sweeps, he lowers his base and offers submission opportunities and by defending your submissions by posturing up, he lessens his base and can be more easily swept.