The billy owner will rub a cloth round the head, nice(?) smelly areas of Billy and your little lady for some strange reason should get very excited when she smells it
(Yuk)
But I agree with Jane, 21 month and no sign, esp if Pom as been there, may be the mild autumn, but might be worth that injection.
Depending when you want her kidding, it may be worth getting a billy rag (keep in a poly-bag) and wafting the bag under her nose for a couple of weeks, if
interest she will be interested but excitement may be a better word? Should at least show if she's the 'silent' type?
If no luck there as Jane says, vet bill! and have the male on standby!