In this way you may actually sort out what is the stumbling block. For example, you may have compressed some aspect which needs to be unpacked or you may be actually missing some crucial element of the idea which you need to discover before you can continue. Sometimes, the linear nature of writing masks a non-linear network of ideas. If this is the case, you could attempt to present this to yourself visually in a concept map or diagram and then write from this. One last thought is that talking it out with a friend can help you identify where the difficulty lies.
Writing is a complex and slow process.
Expecting it to flow effortlessly because you are writing about something you know and understand well and for which you have done the work is a mistake. This kind of mistaken belief can rob you of your confidence which, in turn, further turns you off from writing and compounds the cycle. However, it is also true that you can be a brilliant researcher but, if you don't communicate what you have done to other people, then you may as well have been doing it for nothing. If you are moving from the stage of doing mainly research to concentrating on writing, you may need to develop a new routine. Some professional writers set up definite daily routines to achieve their writing aims.