"One person I've talked to said we hadn't had cold enough winters, and there was nothing I could do about it,"

LOL!! Well, that's a new one :-)) That person must not be at all familiar with hydrangeas and how they bloom as it's not ever a case of the winters being not cold enough - these plants grow and flower in such mild winter climates as SoCal and the Gulf states, even parts of Mexico - but it could certainly be a case of the winters being too cold.

The vast majority of bigleaf hydrangeas set their flower buds in late summer/early fall for the following year so the winter temperatures really have nothing to do with bud formation. But severe winter temperatures can destroy these latent buds, as can late winter or spring freezes. So cold winter temps can affect how well these plants bloom and you can do something to prevent this cold damage. Many folks in colder zones (below 7) will provide winter protection for their hydrangeas to ward against cold damage. If you search through the older threads on this forum, you will get many suggestions and methods on how to accomplish this.

And since we don't know where you are located and if cold damage is the reason for lack of flowering, I would also add that pruning at the wrong time or too severely can also have the effect of removing the latent flower buds.

gardengal48, I'm in Kansas, zone 5a, I believe. I know bigleaf hydrangeas can bloom on old growth, but these have always bloomed on new. In the spring, the old growth never greens up, it's just dead. But this year--to make sure I wasn't causing the problem myself--I made sure not to cut off the old growth. So it's still there, dead, among the new growth. No blooms. I have not pruned anything for a while.

Many of the 'bloom on new wood' hydrangeas are very late to develop flowers on that new growth, particularly in colder zones. I see comments on these forums about them not showing any signs of buds until quite late into the summer, resulting in flowers that may or may not develop before the first frost.

Have you tried any winter protection? That may help with both the old and new growth developing flowers in a more reliable and timely manner.

gardengal48, No I've have not tried winter protection and I'm certainly willing. It's just something I never needed to do in the past, so it seems unlikely. Mine were usually in full bloom by now, with buds being evident by early July at least.

Any chance that they have too much nitrogen? Have you done soil tests in the last 15 years? Unless the soil is defficient in minerals, they rarely need much in terms of fertilizers. But "too much" fertilizers can increase Nitrogen levels, which produces nice, lush, dark green leaves and no or few blooms.

Good idea. No, I haven't done that recently. I rarely fertilize (which I thought might be the problem), but this is an interesting thought.