I don't think that spraying your plants with compost tea would have any effect on the size of your clematis blooms. Perhaps the issue is Messenger which I know nothing about!

Have the same problem with small blooms on my Niobe. She gets the same treatment,food,etc. as my Jackmani who is huge, has huge blooms. I think she is a more petite plant but I've seen wonderful pics on this forum of much larger Niobe blooms. She intermingles with a climbing hydrangea-does this competition suck up too much of her nurishment (or water,hmmm...) ? I don't use messenger Altosax so I can't blame that. Healthy just small. Hope you get more ideas that we can both try.