1) What does your "crack" exactly mean? Does it mean that you saw some void volumne formed in the column?
2) If so, then I think your problem is that you used hexane for the "slurry method". This is what the textbook says but, personally, I think this is wrong. You should use the eluent mixture for the "slurry method" Under most cases, you won't see crack. The only exception is that the interaction of your sample with silica is exceptionally strong and there is still large heat given out. But this is rare.