Al my problems seem to e about the same issue---ie not flowering---this year I started a dahlia patch from some tubers bought on the cheap.
To my surprise they started shooting & are now fine healthy plants flowering a variety of colours---however I do notice that some buds fail to open & seem to go rotten instead of flowering---why is this???
Are you sure they are new buds and not old flowers? Once the petals drop from dahlia flowers they look very similar to buds but are wet inside and do sometimes rot instead of forming seeds. Apologies if you already know this!
Was going to say the same. If they are old flowers you need to deadhead these to keep the plant flowering. Also dahlias do not need a lot of watering. I think if overwatered the buds may rot.