Without seeing it I would say that if you have blocked it several times now (and assuming you are doing it correctly) and are still seeing it then yes you are getting very thin in that area and you may need to do what I said in my above post. I would imagine that you got the sag in the first coat and that is why it is being difficult. That's the only thing that I can think of if you have blocked it several times and still see it.
If it was me, I'd sand with 400 wet, the area where the sag is extending out a few inches around it. Probably do that until I saw some color(base). Then clean it, mask it, wipe it down, and spot prime that area, letting it feather out just a bit into the surrounding area. Block the primer and spot some color in, then re-clear the panel. Probably not what you want to hear but probably the most prudent way to proceed.
I will say that re-clear only may work as well but not being able to lay eyes on it, the safest thing for me to recommend to you would be what I suggested. I've seen guys trying to get a run out then reclear it and you can see the area that the sanded will show up as a small dip or low area.
Keep asking questions if you have them.