There are a number of reasons why oiling out occurs when attempting a recrystallization. If you are using an antisolvent it could be that you added too much of this too quickly, forming a solution that was too supersaturated to crystallize. Sometimes the presence of residual solvents can result in oiling out. I would recommend assaying your acetanilide to determine the level of residual solvent (including water), and then screening different solvent systems than to one which resulted in oiling out. A general rule in crystallizations is to generate supersaturation slowly for best results; this can be accomplished by slow addition of antisolvent, slow cooling, or slow distillation/evaporation.