Fertilizer will not encourage them to bloom - that's not exactly its intent. What they need is sun and heat, both of which can be in short supply in summer in this area. If you can relocate them to an area where they receive maximum, unobstructed sunlight and any reflected heat -- along a cement walkway or driveway, by a paved patio, in front of a southfacing wall, etc. - that will help. But even then, no guarantees.

Even in the necessary hot exposure some kinds bloom more readily here, as early as July some years (in Seattle) and others may seldom bloom at all. So, in addition to the right planting location you have to plant the right cultivar.

What metropolitan area are you in or nearest to?

Most of the more established Crape Myrtles in our neighborhood (around Green Lake) bloomed beautifully during our long summer last year, but very few of them bloomed this year since it was so cool.

Here's a good site on Crape Myrtles with information on bloom times: Auburn University College of Agriculture Common Crape Myrtle page. I would look for one with an early bloom time. Our neighbors have Natchez on the east side of their house--so not scorchingly hot, but it still blooms nicely as long as we have a few months of real summer.

In Seattle a body can see multiple cultivars demonstrated at the Center for Urban Horticulture.

My Zuni bloomed well here in Portland. It gets full sun.

Good to hear that Zuni will bloom. I just planted one this season--it's in the corner of a bed where the driveway meets the street--probably the hottest spot in the yard. Even if it doesn't bloom much, the fall color has sure been pretty.

My Zuni is at the SW corner of my house in full sun, my very hottest site...keeping in mind I'm just a few miles from the ocean and don't get the clear sunny days that those of you farther inland do.

No blooms - not ever - and it's many years planted. I don't even look for flowers any more, but it's pretty for foliage, shape, has no insect or disease issues, and I do like the bark :)

Yes, a cool outer coastal site would be just the thing to keep one of these heat loving trees from budding up. But I don't know if that cultivar is one of the better choices for this region.

Did some pruning & cleanup on a property in Laurelhurst this week, driving by the Center for Urban Horticulture parking lot plantings noticed there was one crape myrtle variety that was fairly conspicuously more red (in autumn leaf) than the others. If I was looking for a crape myrtle I'd be interested in which one that was.

Of course, fall color varies yearly with some plants to an extent that the same specimen can produce a predominantly red display one year and an orange one the next.

Some good reds on potted crape myrtles at Sky Nursery, north of Seattle. The best was 'Natchez'.

My Zuni is now looks horrible. It had not turned color nor dropped it's leaves when the Arctic blast hit in November. I hope it lives, it is close to fifteen years old...It does get Southern exposure and radiated heat from cement driveway and walkway.

Lots of brown or brownish leaves all around up here, many trees and shrubs did not drop them before it turned cold. Probably due to the cool periods this spring and summer. Doubt anything is wrong with your tree, it's already lived there for 15 years. Even if it turns out there was some tip die-back it should be able to sprout from below the damage and bounce back. An exception would be an aggressive Pseudomonas infestation entering stem wounds and killing branches. Have not noticed this on crape myrtle or seen it mentioned.