Hello Pedro! Thank you very much for following my work. That is always a humbling thing for me to hear.
If grown from seed, Ume can be very unreliable when come to producing flower buds. There is a specimen Bonsai Ume in the garden here that is 125 years old and some branches on that tree have never flowered!! Of course, this specimen has never been grafted. Is your Ume tree young? Is it healthy? Usually these are the main reasons a tree is not producing flower buds. But young and vigorous trees can be easily grafted with cultivars that flower reliably. The best time to graft is early spring. We do side grafts with a baggie covering the graft, we put a bit of sphagnum inside to keep the moisture up. White flower types are usually used as under-stock for grafting red flower types, but I have seen the reverse done as well. Just depends on your flower preferences. In Japan, red flower types are popular because they ramify better than the white types. Older trees can be noticeably hard to graft, and it's almost a requirement to get them strong prior to grafting. A 50% success rate is pretty good on them, 75% means your grafting skill if awesome! I want people to loose the fear of grafting their trees. It is a fun, easy way to improve the quality (branches, flowers) of your Bonsai. If grafts fail, no harm done. Trees heal and we can always try again next year. Good luck and I hope you consider grafting your Ume!