Other than a weak firing pin spring, there are a couple of other things that can cause failures to fire in an old rimfire rifle. In no particular order:
Have you checked the condition of the chamber edge where the fring pin strikes. If the gun was dry fired, there could be a dent in that area and without the rim being supported properly you will get misfires.
Also, there could be a headspace problem resulting from a worn locking surface on the bolt or the receiver.
Another possibility is that the firing pin has struck the barrel often enough that it is flattened on the end, spreading out the force of the blow.
As for dirt in the bolt, I suggest that immersing the bolt in a good solvent and really sloshing it around could be tried before taking the bolt apart.