First, I would suggest you to set up a control in which the primary Ab (any other Ab as long as it's from the same species) has been verified to work in your applications. This is to make sure your samples, staining procedure and 2nd Ab are OK.

Second, how are you sure your primary Abs are good? If you're sure they are good and your control works well, then you probably try to prolong the primary Ab incubation time to overnight at 4 degree. 

Thanks Yanqing, Primary antibodies worked for IP and WB here, and secondaries have been used for many other staining in our lab so I'm pretty confident about them. The same primaries have been cited for IF fiber typing in literature quite frequently so I hope/expect them to work. 

Tissue quality might be a potential issue but it worked perfectly well for SDH, shows DAPI and Bungarotoxin nicely.

I incubated BF-F3 overnight actually (SC-71 and BA-F8 were 2 hours each) so I should see something for BF-F3 at least. But my tissues are showing nothing even with long exposure.  

There might be a problem with the fixation in 4% PFA masking the antigen epitope. You could either try not fixing or using an antigen retrieval kit to unmask the epitope agaig. Good luck!

Are these cross-sections or whole single fibres?

Thanks dear Einar, these are cross-sections. For single fibers I run the SDS-PAGE. 

But this is excellent idea, I should play more with the fixation and perhaps permeabilization to see if that helps. Antigen retreivel will follow then.  

Pay my warm regards to Jo. :)