For the second consecutive year my clematis Pamela Jackman, while still putting on the usual masses of stems & lush foliage, has declined to flower. It has flowered in the same (unchanged) spot for over 10 years – I don’t fertilize by the way. A companion clematis (Pilu) is not flowering this year for the first time but a neighbouring clematis – Multiblue – is flowering its head off! No noticeable frost damage; soil moisture same as usual; not in any shade still; no new plants added or removed; no shrubs particularly close. What’s going on?? (pic is from 5 years ago)