In my opinion, if you're interviewing well (independent of your coding skill), 'getting stuck' is something that can't really happen.
I say this because you should constantly be articulating your thought process. Strive to always say out loud what you think your options are and whether you think they'll work or not.
If you say something factually incorrect your interviewer will generally ask you deeper, more specific, searching questions until you realise your mistake or (s)he discovers something you didn't know, or misunderstood, and corrects you so you can move on.
If you rattle through what all your options are and none of them will work, and you know that none of them will work, then there's clearly something you just plain don't know. Then your interviewer will generally prompt you by showing you an analogous example and coaxing you to use it in the question (s)he asked. This is the closest thing that should happen to 'getting stuck'.
When I was interviewing, in this latter scenario, I wouldn't constantly be poking you to find out if you really understood my explanation. At this point I've ceased to care about how much you know and started to care about how much fun you are to teach. Some pointers on how to behave if/when this happens:
If you don't understand, for the love of all that is good in this world, ask a question. Bonus points for precisely-worded questions.
Never claim you understand something if you don't. That's one of my personal pet peeves and I'd imagine most teachers hate it as well.
Repeat and explain things back to me in your own words. It's the single most time-effective way for me to gauge how well you understood what I just said.
Be curious. Find out what the limits are of this new concept. Guess at what the edge cases might be and ask about them.
If after this process, you totally understood, picked up the pieces and smashed through the original question, awesome! If you didn't, well, chalk one up for experience, adopt a healthy growth mindset, and move on. Some folks particularly impressed me by saying at the end "Hey I feel like I didn't do too well with the question about ____ and sort of struggled. Could you give me some pointers or links that I could read up on and use to improve?" The answer is always yes. You might not be accepted immediately but with some willpower, tenacity and grit, you will. And then we'll swap high-fives and welcome you to the clan.