Have you tried opening the fasteners first? What about opening a sub-assembly first?
Depending on the size of your large assembly sometimes the master reps just can't be opened. Do you need to open the master rep? Can you make a simplified rep that gives you want you need?
Are you saying that if you open them first successfully and then open up the top level assembly they still don't retrieve? That shouldn't happen as they will be in session. If that is happening then I am guessing you have a modeling failure at your top level assembly that is causing the placement to fail.
Just out of curiosity, why are you saying the fastener retrieval is the problem? How long is too long for the assembly to open?
In general, usually when there is an issue with a large assembly that should open, it is usually modeling issues such as lots of failed features/components or sometimes it when users have too many assembly features that are set to "automatic intersection" or sometimes large patterns that aren't set up correctly. There are a lot of potential reasons.
Another possibility is you system is running out of RAM and it is working with the pagefile swapping memory. You can check this by monitoring your RAM usage in task manager while you are opening the assembly to see if you are approaching 100% RAM usage.