Unproductive in dissertation research (mathematics/statistics)
Some problems are just hard, and insight can't be scheduled. So, yes, others have had similar problems. Good problems, by the way, are hard to find. They can be too easy and have no real significance. You don't gain much by solving them. Others are just impossible, possibly theoretically, or possibly just at the current state of knowledge. 

My field is different, but one thing I'd probably think of trying if I didn't understand a function would be to model it computationally. Another thing I'd probably want to think about is my partial results. Are they meaningful? Can the be used for other things?

But the best help will probably come from your advisor. If you can explain what is blocking you, maybe the two of you can find a crack to exploit. Just trying to explain your problem and your issue can sometimes free a block, if that is what it is.