then keeping them in the greenhouse over summer.
The bulb doesn't rot just sits there doing nothing.
Sounds like they were too warm in the greenhouse, if your greenhouse is like mine temperatures can get very warm (80°F+) hence me putting them outdoors.
What I forgot to mention is they want to be well fed before they are allowed to die down so that embryo buds form for the following years flowers. Shop bought plants are often forced so any fertiliser is quickly used up and needs replenishing.
I find apart from what I mentioned above I just leave them to their own resources that is:
I leave them in their original compost where they use up any feeding and moisture that remains in the compost.
Once I have re-potted them and put outdoor I leave nature to look after them meaning; if it rains they get watered, if it doesn't they go dry.
The bulbs are quite fleshy so they can be a bit like succulents and store enough moisture to sustain themselves.
When they are fetched indoors then it is up to me to look after them in terms of feeding & watering.