Is being left on read rejection?

Look at the thread before you decide what the read receipt means. If she was replying quickly and asking you things back, one unanswered message is probably just timing. People open a message between meetings, plan to answer properly later, and then the day gets away from them. If she was already giving short answers and not asking anything, the silence fits a pattern that was forming anyway. Either way, one unanswered message isn't enough to read a verdict into. What you're looking for is whether the silence is a break from a conversation that was alive or the end of one that was fading.

Also notice what your last message asked for. A question with a clear, easy answer gets replied to more often than a statement that needs a reaction. If you sent something that could only be answered with effort, that may be the whole reason. It doesn't mean she thought less of you.

The follow-up that fits here is light and forgiving. One option is a new, low-effort question on a different topic, so she can answer without feeling she owes you a reply to the last thing. Another is a small observation about something you both could relate to, with no request attached. A third is just sending something you'd have said anyway, as if the gap were normal. What these share is that they don't mention the silence and don't ask her to explain it.

You can tell a draft works when she could reply in a few seconds without deciding how she feels about you. If it asks for a decision, a clarification, or an apology, it's the wrong shape. Leave out anything about how long it's been, any joke about being ignored, and anything that makes the message about the two of you instead of something you're both interested in.

Send the follow-up once, at a normal hour, a few days after the original message. Sending it late at night or the same afternoon makes you look anxious. A couple of days is long enough that it reads as you moving on and simply continuing. If she replies, match her energy and let the conversation find its own pace. If she doesn't reply to the follow-up either, don't send a third message. That's your signal to stop, at least for now.

If you do stop, reopening weeks later can work if you have a real reason, like something you both mentioned or a plan you could suggest. Otherwise let it go cleanly. Two unanswered messages in a row is a clear answer, and pushing past it usually makes things worse, even if she was only busy.