Besides James C’s excellent point that Lottie usually isn’t looking, there’s some other factors at play that explain it.
One is familiarity. Lottie is this new element they’ve probably only vaguely heard of. Claire is someone they’ve been pining over for some time and as we saw with Ed in Giant Days and his crush on esther, that kind of pining can be VERY hard to break. I’ve been there, you can tell yourself it’s a bad idea all you like, and the Clarem certainly tried last time, but it can take a lot of time to get over it. Sometimes you can flip through crushes like the telly, but it’s probably harder when the possible new crush is your old crush’s best friend.

The second is approachability: Claire is approchable: friendly to anyone she meets who isn’t obnoxious to her first. She’s got a sweet quality and if I was their age i’d be awkard enough to assume automatically kindness= I have a remote shot. Really.. Claire is just very nice. In contrast with Lottie there’s both a high chance of rejection and of spoling a maybe with Claire when really it’s a hard no from both, Claire’s just caught in that classic social trap of not wanting to break these guys hearts but not returning their affections either. It’s like her not bailing out of her earlier degree to please her parents: She KNOWS what the right path is, but it might upset people she cares about. I do feel she geninely likes the Clarem as friends, she just has no idea how to get out and unlike last time, when Lottie was geninely useful for this, Lottie has instead chosen to force the issue in a way that’s going to hurt everyone involved.