From my experience (and I've been reading many research papers for a while), it's rare to find a research paper where you fully understand everything in one go, especially if the research paper was published or released recently or a very long time ago (because, back then, maybe people had a different writing style, used a different notation, or something like that), unless you are an expert on the topic, which is probably not the case, unless you are doing serious research on the topic (i.e. you're doing a Ph.D. and beyond; in that case, you probably don't need to ask questions on this site: hopefully, you have a qualified advisor to whom you can ask these questions!), or the paper is really easy and does not contain any formulas.
Of course, if a paper is published, it must contain something novel, so that something novel could be one of the things that you need to spend some time to understand, but the hardest parts of a paper could also easily be the prerequisites (i.e. the concepts that the paper builds upon), because you may not have a very solid knowledge of those topics (as you probably have already experienced).
There are at least three ways to proceed when you are stuck because you don't understand something
- If you can ignore what you don't understand (i.e. you don't need it for your purposes because e.g. you just need to have a high-level understanding of the topics), ignore it (really!!)
- If it cannot be ignored (e.g. because you really need to know all the details of the paper because e.g. you need to give a presentation at your university), try to understand what you don't understand by picking up a resource on that topic that you don't understand, then read it; spend the time that you think is opportune (i.e. do not spend 6.5 days to understand a detail of a paper if you only have 7 days to read that paper and prepare a presentation or whatever you need to do)
- If you can afford it, stop reading that paper and go back to the basics.
In general, learning is not an easy process and, more specifically, reading research papers is not the easiest reading (because research papers are typically concise, i.e. there's a lot of information compression), so do not expect to understand everything of a paper in one go. In fact, the paper How to Read a Paper by S. Keshav, which gives you some guidelines on how to read a paper, tells you to read a paper in three steps. For more details about these three steps, please, read the paper!