Abby Anita,
One of the most significant factors for women to orgasm is their emotional state and stress levels.
Obviously your stressed about this problem so getting worried about it isn't going to help. In-fact questing for an orgasm when you have sex could be putting you in the wrong state of mind to orgasm.
I'd stop changing your pills for this reason and perhaps go back to the first if this is why you left that one...
I know the answer "your stressed" is a pretty rubbish answer, its so intangible, such a infuriatingly hard concept to grasp, so unless you can actually fathom the reasons why you are stressed don't worry about it.
Infact take a break & stop worrying about orgasming (Although I understand thats alot easier said than done). Even if its another reason, and you cured it, stress could still stop you, and you could never be aware of it.
By-the-way,.. How long has this been going on?