my question is regarding a surge analysis which I am currently doing; I am working on a relatively "long" piping system (~4 km) which is subjected to surge. I broke down the whole system into 4 smaller calculations in order to make it easier for my computer to run. what really baffles me is that in 2 of my "sub-calculations" where there is no rigid elements (only pipe,elbows and bi-directional supports), CII is unable to calculate all the modes within 50 HZ. it all goes well until about 42 HZ when it starts going through a repetition of modes and it goes on and on. I extended the the frequency array space up to 1000 but still the same problem existed then removed friction from the analysis and the outcome remained the same. if I chose for smaller frequency cut off (e.g. 35) everything is done but in the end I receive " strum sequence failure" warning. needless to say that in my output there are data caused by neither pseudo nor missing mass!
could anyone explain why this happens?

I suggest you adjust one of the settings found in the "Advanced" tab. Reset “no. of significant figures” (from 6) to 10 and run again.
I usually do not fool with these settings but we had a situation similar to yours last month where it appeared that CAESAR II was looping through a series of natural frequencies. By adding significant figures to the frequency extraction, the program could again distinguish the modes and that looping disappeared.