Do the owners have earned income (LLC/parternship) or are they W-2 employees (corporation)?
If it is a corporation, find their pay stubs and confirm if the amounts were actually withheld from their pay. If they were withheld, deposit the amounts now, with lost earnings, and pay the excise tax. If they were not withheld, amend the returns.
If it is a partnership/LLC, so there are no W-2s to refer to, then you have to look at the partners' elections (the partners do make a written deferral election before the end of the year, right?). Then, same logic as before. If they elected to defer the amount but it was never deposited, deposit it now; if they did not elect to defer that amount, then amend the returns.