Hi, welcome to GoY, do you mean actual Mimosa, or the florist's mimosa ?, there are about 400 species of Mimosa, ranging from annuals, evergreen perennials, shrubs nd small trees, all of which are frost tender, and need a long hot summer to flower, if you mean the florists mimosa, which is Acacia dealbata, which is half hardy, can stand temperatures down to 32f, and need to be grown against a warm wall, these want to be a tree growing to between 50 and 100ft high, with a spread of 20 to 30 ft, so if it's in a pot it may have run out of nutrients, they're much better off in the ground, if it doesn't get too cold where you are, also they can flower almost any time between winter and spring, and we have only just gone into spring, so may flower a bit later than in previous years, Derek.