Technically what is happening is called a salary deferral. You defer your salary, and the employer deposits the money into the 401k for you.
Suppose you get paid on the last day of every month, and your December 31 salary is reduced by your agreed deferral. Your W-2 for the year should reflect what you were actually paid, including the deferral, even if the employer doesn't make the deposit until January.
If your employer is delaying your deposits by more than a couple of week, it's time to contact your state employment regulator -- it sounds like they are on the edge, and running into trouble, and need a government regulator to look into their business details. You might also want to think about ending your 401k deferrals, taking your full paycheck on payday, and then contributing to an IRA or Roth IRA. We can discuss those options (which depend on your salary and family situation) if you want additional info.
