Locking Content Navigation

I will try to keep this simple! I have for example a contents page where I would like them not to be able to proceed further until they have completed the previous module. Each module within the course is either a video or a pdf download.  Once they have completed that module, I have set it to jump back to the contents page where the next module state has changed from locked to normal (and by the way the completed module now shows a tick). Nice and simple so far. So looking at various discussions  etc. I worked out what I need to do with states and variable etc...and it works for the first couple (they are videos), next one is now unlocked and showing as normal, but this one is a download...and this is where it seems to fall apart. I click on it, it takes me to the relevant module, I hit my download button, it downloads, jumps back to the contents screen, but the next module is still locked and the current module is still showing as normal. I am now pulling my hair our with frustration with something that is probably very easy to fix!