Make sure the ID of the position you are entering is no longer occupied. I don't mean in anim files now, but in body.def. Furthermore, it is important to realize the higher the client version, the less chance of a free slot for new animations. What kind of client do you use? version?
The only other thing I can recommend is try every empty L slot before giving up, for some reason I remember there being a few L slots that I found to work, quoting a memory from 10 years ago!
Post automatically merged:
Other then that, you could replace a animation you no longer want to use from the Vanilla UO L Slots