If you were expecting to breed on that second heat it can be frustrating. Also mine tended to start super amazingly late which was a little concerning at the time. 18 months in a toy breed is nearly unheard of yet mine started at 18 months and 20 months and didn't cycle again until 9-14 months later.
I don't tend to worry too much about missing heats - when they happen they happen. If you're trying to plan I suppose you can visit a repro vet to find out what's going on.
We have no plans of breeding yet, maybe around 2.5-3 yr after titling/testing, etc but it's just odd and yes, a bit frustrating.
Sloan came in May for her first heat. She was 1 yr March 7. We excepted another heat in Nov but she's still dry.
I have an intact male with her daily and, well, he's not told us anything. He does a lot of tasting, all the time, which threw us off, but no real signs.
My TD is convinced we missed a heat because she's so lean and active but I'm not convinced, again, the intact male with her all the time (when not unsupervised, then they're crated) but maybe he's just as dumb as we all think he is. LOL
I just want to clarify if I should be concerned at any point. If she turns up sterile, no big, a bummer, but we'll be okay. If it's anything not okay, I'd love to know and or know when I should be anything but okay.