Some can be stiff and not every set of connectors fit 'nicely' and need some force. Before ramming them together, double check the ends and make sure they're lined up properly. Molex connectors are 'keyed', they're not entirely rectangular. Two of the corners are 'shaved' so it only fits one way. Provided you have the angled corners of the male plug lined up with the beveled corners of the female they should fit.