Reasons for bud blast include, although not limited to; lack of humidity, cold/cool draughts, poor feeding regime, overwatering or too wide a temperature variance.

As this has been a gradually developing problem, they may need to be repotted. Mixes and methods will vary with the species, but there are some constants. In general, they need pots just a little bigger than the one they are in now. Like almost all plants, don't plant them any deeper than they are now. Plant them in moist (not soggy) mix, and let them sit several days before watering, and a couple of weeks before feeding. The best time is right after a bloom cycle ends.

You might also make sure that a tree or shrub isn't overgrowing their window, and gradually reducing their light.

Thanks guys x