Major issues with cash balancing at the end of session

[Beau-Degat]

As we have for a couple years, at the end of the day in our brick and mortar store, we use the the counting tool to count out the cash and end the tracking session. Often there will be small discrepencies, but nothing over say +/- $10 in a day (sometimes up, sometimes down -- thats just the way she goes, right?) no big deal.

For a little over a week now our humble little cash register has been wild. with discrepencies that are often over 15$ and sometimes over $100, which is a big portion of the cash in the register at the time (think like 30%-40% of total cash). This is baffling to us as it's just not possible. One person will be stanging in front of the cash register all day, no thefts. They'll be giving out accurate change and would notice, as well as the customers, if they were giving the wrong change. 

We're getting to the point where we dont understand what's happening! Is there a bug or something we're doing wrong?!

HELP!!