What is the plant like overall? Is it as vigorous and robust a grower as in the past or is it lacking in the amount of vines it has put out? Have you treated it the same as in other years as far as fertilization and pruning? Any other differences you see other than the number of blooms?

Isn't it a little early for Jackmanii, being a late flowering clematis?

Depends on the exposure the plant is in Janet. Plus the OP probably knows when it has bloomed in the past and is gauging his/her question on that? However if the plant doesn't normally bloom this early, the few flowers it is throwing out now could very well be teasers for a bigger show later in the season!

Hmmm.... then you really can't trust the books on bloom time either. lol