I have not really heard of any person in Pa-Auk having trouble extending the sign.
I have observed many thousands of interviews while waiting for my thousand or so interviews with Pa-Auk Sayadawgyi. (usually one can hear 4 to 10 interviews while waiting). This is while waiting with ven Pa-Auk as a teacher alone, not to mention other teachers.
Usually (as I summarize thus what I have heard)… usually the monks who get first jhāna can usually attain all jhānas. Most… but not all. Usually going from successfully attaining first jhāna to successfully attaining 3rd jhāna is fairly quick (a few days or one to two weeks). There are some who struggle with 4th. (this is anapana). The issue is the “were you breathing?” question which gets asked. Often the accepted answer is “If I pay attention, I’m breathing. If I don’t pay attention, I don’t know”. But usually and I say this lightly… they know they were not breathing after they emerge from the 4th jhāna meditation. After they get past this… attaining 4th jhāna in other objects is nearly the same. I’ve never heard of trouble with kasina if anapana was successful and they can see body parts using the light of wisdom.
Then most seem to move on to body parts. If they can see body parts (and most can), they often do all 4 jhānas on white kasina… and usually also succeed on the arupa jhānas which are an extension of 4th (5th abhidhamma jhāna). Generally speaking.