Why am I not being copied into the replies?

[ChooseYourUsernameWisely]

I work in a small admin office, and one of my colleagues has recently lost her mum. My colleague emailed our Supervisor to say ‘happy to contribute…let me know how much …’ copying myself into the email.
I was off work last week but saw there was no reply from the supervisor but my colleague had received a reply.
This has happened before, me left out. I’m part time 18 hours and my colleague is full time. The supervisor is hard to get hold of the best of times, but surely replying to both of us saves time!
Any ideas why she would do this and what can I do about it?

To be honest I quite often reply only to the person who sent the email rather than reply-all, unless it's something obviously applicable to everyone on the chain. It's just good email etiquette to avoid reply all, as otherwise inboxes get clogged with lots of stuff especially if your colleagues are addicted (as some of mine are!) to copying every man and his dog into trivial communications. This is particularly true where people are on holiday so you want to avoid them coming back to 100 email long threads about something that is now entirely finished and irrelevant to them. So I doubt your supervisor is doing it on purpose but it easy to habitually hit the reply instead of reply all button. I think if the email was worded as you've said it, ie 'I am happy to contribute, just let me know" (emphasis mine) I would definitely have just replied direct to your colleague and not copied you in. A simple way to solve the problem would be either for you and your colleagues to be clear when an email is meant to be collective and use plural pronouns, so in your example I would have written 'Chooseyourusername and I are happy to contribute, just let us know' and then it would have been clear to reply to everyone. Or if your colleague had forgotten and just written the email as though it came just from her, I would have replied saying 'Me too please' (thus disobeying my own etiquette rule of not sending extra inbox clogging replies of course ). If you and your colleague frequently have to share communications and it's becoming difficult to manage, perhaps look at setting up a shared mailbox (easy enough to do on Outlook) and send/receive things that are relevant to the whole team on there so everyone can see (you can keep your own mailbox for things just for you?