Variety.
Some varieties are slower growing and take longer to establish mature flowering habits.
Too close to trees, shrubs or overhanging branches.
Too crowded.
They need one square metre (1 square yard) of area to themselves.
Wrong aspect.
They need at least half to three quarters of a day sunshine.
Poor drainage.
Waterlogging of soils is most likely to happen in winter time. This will encourage root rots. Do not overwater; they do not need it and especially don't water over the summer after flowering.
Roots too small or poorly divided.
They usually survive but will take one or two years longer to establish.
Roots too large.
Transplanting large whole clumps is not recommended. 3-5 eyed divisions with at least one tuber (storage root) 15-25 cm long are considered optimum.
Planted too late.
The optimum time for bare root is autumn, and winter is ok. They can be planted in spring but the growth will be poorer in the first growing season. Again, it will usually survive and come away well in the second season. Potted plants can be planted during the growing season.
Blackened shrivelled buds,
either due to small plants just getting started and not having enough "energy" to develop a bud, or Botrytis, a fungal disease. Botrytis can be controlled by
- cutting down and removing dead foliage in autumn
- spraying Captan, Euparen or Rovral (=Defence) in spring e.g. 2 weekly intervals, perhaps 2 or 3 times
- judicious use of composted material, which can introduce fungal spores. Keep it away from the crown of the plants.
Excessive cutting of flowers.
Once established, usually third year, cut no more than half of the stems.