2014 E350 doesn't reverse
 

Recently puchased 2014 E350 with about 57,000mi, once in April when placed in reverse, it didn't move. Seemed as if in neutral, placed back in Park and into Reverse n same, so placed in Drive and went on. It hadn't happened again until today, with about 61,000mi, same but on second try this time it did reverse. Gave a good amount of gas and indicator for Reverse is lighted as well as display showing Rear Park Aid, but just didn't move, until lever back to Park and then to Reverse again. What could cause this?