Troybilt horse

Hello, my troybilt 46” horse, Model 13AT78BT023, would not turn left, YouTube showed need replace steering shaft so replaced, replaced and still same problem, finally found it was grass accumulated in steering gear plate ,cleaned and turned ok, then after mowing few minutes the steering would not turn wheels, the bottom steering shaft screw came loose, tighten it up and again same happened, put old steering shaft back on and same problem. What have I done wrong ? I don’t understand why the screw at end of steering shaft keeps coming unscrewed, hadn’t done this before.