Tbh it would entirely depend on the situation as to whether it was worth being bothered about.
Firstly if it was a one off thing (eg concert tickets for a sold out tour of your favourite artist) vs a casual trip to the cinema to watch a movie you'd all expressed interest in because they were both free one afternoon. If it was a casual trip I'd be a bit miffed that they didn't text but I'd get over it. If it was a bigger event then I'd feel really left out and would have to say something.
Secondly, you dont have to do everything together. I have a group of around 4/5 friends that I have known for 20 years. We have regular nights out all together but it's not always easy to find a night everyone is free so it is perfectly normal for groups of 2/3 to get together without the others purely down to scheduling.
If it was an event that cost money and I'd told my friends I was skint that month (definitely my situation in January as I work in retail so no overtime, hours cut back and recovering from Christmas) then I could see why they might just go without me without mentioning it so I didn't feel bad about not having the cash to join them or feeling obliged to offer to sub me all evening.
Maybe they bumped into each other and someone mentioned the event so it wasn't organized as such, so they didn't think to invite you.
Have you been distant recently? I know that sometimes my friends won't invite me to things if I have turned down other recent invites due to previous commitments and they leave it to me to get in contact when I have the time. For example since the end of January I have been busy - DD has had a lot of activities and overtime has been reinstated in my store so, after only doing my basic contract for the last month, I can't afford to turn down overtime (esp as my store will give priority to those who always accept overtime before they ask people who turn them down regularly) as I need the money more than I need a night out.
Have you been ill or under the weather? Whenever I'm ill I will let friends know and then I will be the one to reach out when I feel well enough to socialise. If it's a physical illness people usually wait for you to tell them you're better and up for socialising. I also have anxiety so sometimes I just need to withdraw from social activities to sort my head out. When that happens my friends will keep in touch but don't invite me to things as they dont want to put that social pressure on me until I tell them I'm ready for it.