One of my bitches has not come into season for over 18 months since he last litter . She has regular contact with my other girls who’ve been in season and my boy who she is usually mated with . Any ideas on what I can do to bring her into season ?

It's really best not to use anything and just to wait.  What was the usual interval between her heats?

Prior to this she’s been fairly regular every 6-8 months. She’s is good health otherwise , has good food and lintbells oil daily so shouldn’t have any deficiencies

When a bitch has a litter, it can upset the cycle a bit, but that's a touch extreme (and did she had a litter the last time?).   She may had just decided enough is enough (seasons) - I'd get her spayed to be honest.

First off, see your vet.  They can run some bloods to check hormone levels and that might give some indication of what's happening with her cycles.  You can tell from progesterone levels how long ago a bitch was in season.

Don't forget that silent heats do exist and perhaps she had a heat but you didn't notice??

If not, an ultrasound and check of the uterus and ovaries would be the next thing. 

I can't really understand why people would be telling you to spay her.  It's like people think only their own dogs are worth breeding from and everyone else's are just not worth whatever the effort would be, in working out what's behind a problem.

> I can't really understand why people would be telling you to spay her.  It's like people think only their own dogs are worth breeding from

I think it's more likely this..

> and my boy who she is usually mated with

If she is usually mated to her own dog, she's probably donated enough to the gene pool by now? Repeat matings are fine where the progeny are exceptional and/or a small litter, but 'usually' using the same dog?

Oh right, totally agree in that case chaumsong.  I missed that sentence.