Buckling Behavior of Shells

Hello community,

i'm trying to evaluate the mechanical behavior of a shell structure. For this purpose i'm running an AnalyseTh.II.O and afterwards an analysis BModes for getting the buckling modes and the buckling load factor.

As my shell is basically parameterized by two variables, i'm searching the optimum combination of these two regarding buckling and elastic energy.

For 95% of the combinations tested, the buckling analyse is running without any issue and is giving back the critical load factor. There are some cases in which the structure buckles and i'm getting back a load factor 0. But for a small number of combinations, the analyse is running without giving back any value at all.

I tried to change the mesh density which solved the problem for a few cases but not for all. Any ideas what's this problem due to?

I would really appreciate any help. 

Thanks a lot