Simple reason for a squib, and the only one I can think of, is your load is too light. Really light. I load .38Sp with a mid range load and shoot it in a Model 52. I load cases over and over until they split. I've accidentally loaded a split case or two not noticing until after they were loaded. I shot them anyway, usually in a revolver, and tossed them out afterwards. Never a squib from a bad case. Light or no load is the only cause.