Oh, I thought it was attaching to a rock in the back, not just floating.. that is different.. and not good. A healthy anemone should attach to a rock in a matter of minutes. If I have a "free floating" anemone I would cup my hand over it and gently set it on a rock and hold it there for about 1 minute.. healthy ones would always grab onto the rock within that time. Normally it wouldn't stay there though and would end up walking around the tank until it found the spot it liked.
If you have a big enough fish net you can put the anemone on the rock and gently set the net over it for an hour or so to see if it will attach.