Failure to deduct mandatory employee contributions

I received a call today from a client.  They maintain a DB Plan with mandatory employee contributions of 3% of salary.  It appears that HR missed this on a small number of employees and has realized it now.  This has been going on for 3 years on one employee and about a year on a few more.  The total number of employees impacted is less than % of the total population. 
 

Does anyone have thoughts on an appropriate correction?  This isn’t a missed voluntary deferral situation where we have set guidance on the correction.  Does the employer have to make the missed contributions?  Can they work with the employee to “catch up” the amounts that were missed?

Thanks for any thoughts!