Riding his 300cc Sherco factory trials bike, Fox (20) said he struggled to get accustomed to the more powerful machine as usually he competes with a smaller 125cc engine.
''I was not confident in my riding,'' he said.
The Labour Weekend event was won by world No 13 Jack Sheppard, of England, ahead of seven-time New Zealand moto trials champion Jake Whitaker.
Although it was a low for Fox, he has achieved some highly creditable international results over the past months during his second year competing in the world trials championship.
Undertaking a trip to Australia and two separate tours through Europe, he competed in eight of the championship's 13 rounds.
He finished sixth overall out of 25 riders that scored championship points - consistently earning top-10 placings at the events - and hugely improved his ranking from 2013, which was 24th for the season, with his highest placing a 14th.
Fox then rode in the Trials Des Nations in Andorra in September as part of a four-member team against 16 other countries.
The team improved three places on its 10th position last year.
''The whole team rode really well and worked together along with our awesome three minders, my father Gavin Fox, Kevin Gundry and Mike Skinner,'' Fox said.
