Friendship advice

I sent a message to my friends’ and I’s group chat at 3pm asking if we’re up for our weekly social club tonight and one of the girls got back to me at 3:20pm saying she’s got plans so can’t make it, which I said was fine. However, the other girl read the message at 4:50pm and hasn’t responded whatsoever, so I’m in the dark now about whether I should bother asking her privately or just letting it go? It’s 6pm now and I haven’t heard back from her, so I’m assuming it’s not happening? Or should I assume the reading the message and not replying means she’s going? Advice please.