Louth Town failed to build on their derby win at Market Rasen when they were beaten at CGB Humbertherm on Saturday.
A hat-trick from Jack Boswell did most of the damage for the hosts in a 4-1 win at Fulstow with Callum Hyland on target for Town.
The defeat, against a side they had thrashed 5-1 in the reverse fixture in September, left Town with just one win in their last six Lincolnshire League matches.
Louth remain sixth despite the reverse and will have a chance to improve their form at home to Sleaford Sports Amateurs on Saturday. Kick-off is 2pm at Saltfleetby.