I have been successful grafting guavas. The only success that I've had was using the veneer graft method. That's it. Nothing else has worked for me. Also the freshness of the budwood is critical. I've had the best results when the scions are grafted the same day they were harvested. It seems that once they take, they grow fast. I did not document the time of the year that I performed those grafts. The last successful graft that I performed I think it was in the middle of the winter. It was Red Malaysian Guava and is pushing already. One more thing, it seems like some of the grafts take awhile to push as well. Hope that helps.