I have a collection of fuchsias, which are marketed as non-hardy so I have overwintered them dormant by removing all the greenery, pruning them hard and keeping them in the shed. I am in the UK and the winter has been quite mild but with a few months of fairly hard frosts. I brought them out around the beginning of March and one had died, several others are growing back nicely with varying vigour, but two have yet to make any progress. I can see that there is green in the stems near the base of the plant and they have started to swell and slough off the old bark, so they are definitely alive, but there is no trace of a shoot on them. One of the planters is small enough to fit in my cold frame, so I've put it in there to give it a little extra warmth. So far, no good. There was actually a light, late frost a couple of days ago but the growing green plants don't seem to have been troubled.
Is there anything I can do to encourage them to sprout again? Are fuchsias particularly keyed to daytime temperatures, length of daylight, or something like that which will come around of its own accord?