It's normal, don't worry! Some people happen to find new friends at the school/nursery gates, some don't. You've got children the same age but it doesn't automatically mean you're compatible as friends. Is there anyone you think seems nice who you'd like to be friends with?
Of the people I've met at the school gates, I say hello to several but that's all, chat to a few occasionally (mostly about school stuff) have had a cup of tea with a small number even more occasionally, I get blanked by two not-so-cheery souls, and haven't had any contact with the rest.
The most sociable times have usually been at the children's parties as you can chat to the other parents but also help with coffees or tidying up if no-one else wants to chat. This was more a school thing though, it rarely happened at nursery.
If you would like to get to know people, you could join the PTA or committee, be a helper on outings, go up to people and ask if they had a nice weekend, pick one or two people and ask if they'd like to come round for coffee, invite your DC's key group (plus the parents) round for a Christmas activity etc.
Sometimes you can do all that and it still doesn't work, because you're just not a good match as friends. It's not your fault and doesn't mean you will never make new friends! At least I hope not, because I have made virtually no real friends since living here over 10 years, just a lot
of acquaintances. I've made some lovely friends at other times in life but sometimes it just doesn't happen as we'd wish. You're not alone!