1 of 4 datatables not showing "...loading"

Hi,
I have 4 datatables and all works to my satisfaction. But on the heavy one with 600 lines, it doesn't show "...loading". Thats too bad, cause it's the slow one of the 4 datatables. I've tried to comment out all the extra code on the heavy one, but I cannot get it to show "...loading". It has a lot of calculations in call backs over 50 columns and also some calculations in the php files, so it's not among the fast ones, so the more useful it would be to have the "....loading".
Any ideas as to why apparently equal coded datatables has one without "...loading"?
Best regards
Claus