This is a very common pattern that is largely ignored in the literature as a problem. Its one for which we are always monitoring because it can occur no matter how careful someone is in preparing spores for inoculation. It was this consistent distinction which led us to believe the source of contamination was hyphal fragments associated with the spores separated and cleaned for use as inoculum. Sometimes these hyphal fragments are not detectable, despite rigorous examination of spores. It is this constant possibility of intrinsic contamination that requires strict examination of all cultures prior to take-down and storage.
Some cultures just stop sporulating after several propagation cycles.
There is no Viagra for fungi, though. The former is easier, but we haven't done enough of these to know which might produce better results. The main reason is contamination. We have yet to find ANY cultures that remained monospecific when this was done. All sorts of things can happen in the glasshouse during this time period which could compromise both purity and health of the pot contents.