fall behind in something—
1. fail to do enough of smth. and so move to a position behind others:
- The country has fallen behind its competitors in the manufacture of cheap cotton goods.
2. fail to maintain a schedule of payments, etc.:
- Jacobs began receiving letters from the bank threatening foreclosure when he fell behind in his payments.
fall behind on something —(also: fall behind with something) = fall behind in something 2:
- John fell behind on his car payments so the bank took his car back.
Note: The expression does not correlate in meaning with the phrase fall back on something—turn to smth. held as a reserve for help, etc.:
- They had some money in the bank to fall back on.