There are cases when completed tasks or game offers might not appear as completed. This can happen for different reasons, like if the game was played before the task was started.
Another common reason is that you may not have tracking enabled. You can find out how to enable your tracking here.
This can also occasionally occur if the task is broken. If this is the case, you should contact the task provider. Most task providers embed a support link for this purpose, like the one you can see below.