Above plus we have had these roses about 5 or 6 years and they have always been beautiful. We had a real hot spell about a month ago, but the roses were well watered I thought --drip system
My Red Double Knock Out Rose Blooms Turned Brown and Are Dried Up, the Foliage Is Fine. It Hardly Has Any New Blooms
Also check out this article link: https://www.gardeningknowhow.com/ornamental/flowers/roses/no-blooms-on-roses.htm
It could be that the rosebush is taking a break to get over the heat stress of the real hot spell. I would use a moisture meter and check the soils moisture at each rosebush. I usually check the soils moisture in at least three spots around each rosebush to see what the overall soils moisture is. Sink the test probe down as far as it will go to get the reading too. That way you get into the root zone well enough to get a good read. It may be that the drip system needs to run longer or more often. To help them deal with the stress and shock and get them to blooming well again, hand water them using a large watering can with a product called Super Thrive and a root stimulator product mixed into the same water. Do this the next 4 to 5 times they need watering and it will help get them going again. Knock Outs are pretty tough roses but are still subject to heat stress and shock as well as insect caused stress and shock.