What should I do if a professor says she doesn't understand my questions in office hours?
It's possible that your professor just doesn't want to help you: in that case, obviously, you need to find somebody else.  Or it may be that she just has bad interpersonal skills and comes across as rude and patronizing when she's just trying to be efficient. In any case, I'll assume that she does want to help but genuinely can't understand your questions.

Most likely, it's because you're asking the "wrong" question.  A common phenomenon is the so-called "X-Y problem", where you're asking about the wrong thing. For example, you've tried to solve an exercise but you couldn't do it. There are two basic possibilities: either you used the wrong method, or you used the right method but got stuck somewhere in the implementation.  In the X-Y problem, you start asking detailed questions about how to fix your implementation of the method when, in fact, your problem is that you used the wrong method. There's a great post about recognizing and dealing with the X-Y problem on meta.stackexchange.com.

A related phenomenon is asking questions that only make sense to people who share a particular misunderstanding of some other topic. You don't understand X so, of course, you're asking about X.  However, the root cause of this is probably that you don't understand Y.  Because you don't understand Y, the way you think X works just doesn't make sense: your question only makes sense in the context of your misunderstanding of Y.

Random (and slightly silly) example.  Suppose a student of automotive design notices that the tyres are black and the steering wheel is black, too. He decides that all wheels have to be black so there must be a reason for this and builds up some elaborate theory in his head about how black things rotate better or something. And then he sees that his friend's car has a beige steering wheel. The next day, the student goes to his professor and asks what mechanisms the friend's car uses to compensate for the deficiencies of its beige steering wheel. This question makes perfect sense to the student but is total nonsense to anybody else.

The solution is to work backwards through your assumptions with your professor, to try to find the step where you misunderstood.

Student. What mechanisms does my friend's car use to compensate for the deficiencies of its beige steering wheel?
Prof. Er... I don't understand your question.
Student. Well, cars need to have black steering wheels because they rotate better. My friend's car seems to steer fine so it must have some way of dealing with the bad rotation of its beige steering wheel.
Prof. Why do you think that black steering wheels rotate better? [Note that slight differences of phrasing in this question can make the professor seem much more or less sympathetic. "What makes you think that..." can come across as being annoyed, for example.]
Student. I'm not sure but maybe it's because [whatever]. But the tyres and the steering wheel are both black so there must be reason.
Prof. Actually, that's just a coincidence. The tyres are black because they add carbon to the rubber to make it stronger, and the steering wheel is black because the designer thinks it looks nice.
Student. Ohhhhhh.  I get it, now.

It may be that your other professor is better at answering your questions because he's better at figuring out what the root cause of your misunderstanding is.