While a big update to BBM was rolled out two days ago, some folks have reported issues with BBM Channels wherein Channels weren't fully loading or comments couldn't be posted to channels. BlackBerry quickly fixed the issue by releasing another small update to BBM, but it looks as though some folks never fully got the notification for that release and as such are still running the initial update which has the problems present.
That said; if you're experiencing issues with BBM Channels still, be sure to fire up BlackBerry World and ensure you're on the absolute latest version of BBM. This will fix your BBM Channels issue as well as a few other instabilities found within the app. One other issue fixed in this build – BBM Custom PIN pricing has gone back to $1.99 as well. Sorry folks, that 'pricing drop' seemingly wasn't intentional but if you got in on it at the $.99 price, good for you. It remains to be seen if it will renew at that price though, so keep that in mind.