I've been through this problem multiple times and it only happened when I was accessing Trello with my phone (I use a PC, MacBook, and iPhone to access Trello)
so the comments or my reaction didn't get synced which means my client or coworkers won't be able to see that comment although on my end it appears to have been sent successfully
I haven't found any solid solution to it although I tried logging out of my account and resending the comment (which worked a few times) or deleting the app entirely and reinstalling it
so, is there any solution to that problem hopefully?