Long, sorry...
There's a group of mums at my dd's school, all of whom I get on well with - some a little more so than others, and with most of them I have developed quite good 'individual' relationships.
One of them is actually my closest friend round here - in the sense that our families get on really well, we are godparents to her children, we've been on short holidays together, and regularly hook up for Sunday lunch, coffees and chats.
Another is one who I get on well with, we have children who are close friends, am going to dinner at her house this weekend.
Another I go out with lots, email and text lots, we help each other out a lot, again - children very close.
The rest (about three or four others - it does vary as new people, again who I know - often quite well sometimes - seem to be invited especially when they have new babies) I get on fine with, children same ages, never had any great fall out or anything. There is one, who seems to be a bit of a queen bee, who I get on fine with on a superficial level, but sometimes I really wonder if she actually deeply dislikes me! Not sure why, I guess we kind of lock horns sometimes.
They all started to get together regularly for lunch about a year ago, which tailed the back of a class that they would take their children to. I've been asked along a couple of times, in unusual circumstances (too tedious to go into here) but have never been asked to regularly. It would make sense for me to be asked, because four of dd2's closest friends are there and I don't work on the day they get together. It MUST have passed their minds to ask me to 'join them', but they never have.
Anyway, I've had times of feeling a bit 'left out' before but have tried to just get over it. But today I walked up to them where they were assembled outside the school, made a bit of light banter, then it became apparent that they were all getting together at my 'best' friend's house. I just think it would have been nice to have been asked too FFS!
A bit about me - I'm a little, but only a little, younger than them, I work pt (all bar two are SAHMs so I don't think it's the fact I work), quite opinionated and tbh not massively into talking about children all the time (they all are) - but I try to nod along dutifully and be as agreeable as possible. I have a lot of friends from all kinds of different groups of mums and never seem to be short of invitations and things to do. In reality I probably wouldn't have time to meet with them on a regular basis, but I'd just like to know, have some insight perhaps, as to what it is that makes me 'unworthy' of being asked along to their get-togethers more often.
By the way, I don't smell. 
I'm aware that I may come across as a bit arrogant here but I'm pissed off and actually feeling quite hurt!