I've worked these tasks a lot, and not knowing more details of which tasks - here are a couple possibilities...
First, the tasks almost always have a limit as to how many panels one is allowed to do before it "cuts you off". Example, I like to do the "search for terms" tasks when they appear, and they've been cutting me off around 7 or 8 panels completed. If you had worked a page before getting the message you worked all the tasks available, you should have been credited for the panel you submitted - if not, then chalk that up to a rare glitch (has happened to me once in a while). If you had worked a few panels on that same task a different day, then back again later - that would explain why you were able to work more one day vs the other - that you had not reached the limit on your first day of work, but hit the limit on your next day. The good news is when that round of the task is complete, and it's posted again in a week or so, you should be able to work the task again.
If you try a task and get the "all finished" message from the top - that means either it's still the same version of the task you had worked previously (sometimes tasks may take a couple weeks before they complete), or the if the task available count is low - what might be happening is others are currently working the remaining tasks so it's not letting new people in until those people either complete the tasks successfully or unsuccessfully. If any are unsuccessful, then the task could open up briefly. If this was the case, it's a timing issue - just not catching the task as it's available. I tend to avoid tasks that are getting low as I almost always get the "all finished" message - even though I had not worked it, the task itself has no more work available.
Hopefully what you were observing fits into one of these scenarios and this will help you when choosing future tasks.