Lisanne Beck, 47, allegedly performed a sex act on Simon Murphy, 48, during a Paloma Faith set in September.
The Old Bailey heard the couple were filmed conducting the alleged behaviour, but the jury failed to reach a verdict and were discharged.
The pair, from Swansea, denied outraging public decency.
Judge Peter Rook QC has given the prosecution until Thursday to decide whether to pursue a retrial.
He said: "Whatever the Crown says, it will have been a chastening experience over the last few days, I'm sure everyone will agree with that."
During the hearing prosecutor Tom Cleeve told jurors while "there are some amusing aspects to it... it was not funny at the time" and a 16-year-old girl had to be shielded from what was going on.
He said the two defendants were of "previously good character, but in that afternoon I'm afraid they blotted their copybook".
During the hearing Ms Beck said the film showed she was trying to wake her boyfriend up, while Mr Murphy's legal team said he was completely unaware of what his girlfriend was doing.
A couple accused of carrying out a sex act at a BBC Radio 2 concert in Hyde Park must wait to hear if they face a retrial after the jury was discharged.