When I start a work, after sometime if I try to use dynamic topology and simply doesn’t work.
It doesn’t matter how much I try or if I start over a new work or file
So the only option I have then is to restart the ipad (last ipad pro model) and then open the file again and then the dynamic topology works normally… for sometime.
This sounds strange. It could be hundreds of circumstances but most likely in front of iPad 
Dynamic topology does not necessarily have to produce new geometry if density settings are fulfilled. Using Constant I.e.
Best you make a video with your dyntopo settings and how the problem appears.
Show a screenshot of the app with the dyntopo settings and the tool used.
There’s no known issue, so the problem is definitely coming from a misunderstanding on your parts.