Terry Butcher's men lost 3-1 at Partick Thistle while elsewhere Dundee United came back from two goals down to beat St Mirren 3-2 and move into fourth
Top-six hopefuls St Johnstone and Hibernian fell to Scottish Premiership defeats against Ross County and Partick Thistle respectively.
Sixth-placed St Johnstone had a chance to extend their advantage on Saturday when they faced County after Hibs had lost earlier in the day.
However, the Perth side failed to take that opportunity as Melvin de Leeuw's sixth goal of the season gave Ross County all three points at McDiarmid Park.
That result will frustrate Hibs boss Terry Butcher even more after his side slumped to a 3-1 away defeat.
Chris Erskine gave Partick the lead two minutes before half-time and Lee Mair doubled their advantage on the hour.
Mair's strike seemed to spark Hibs into life and, when Duncan Watmore halved the deficit, a comeback looked on, but the visitors were unable to find a second goal and Kallum Higginbotham scored a late third to seal the win.
Dundee United are fourth after battling from two goals behind to secure a thrilling 3-2 victory over St Mirren at Tannadice Park.
Paul McGowan and Steven Thompson had put the visitors ahead inside 40 minutes before Brian Graham struck his fourth goal of the campaign with 11 minutes left to trigger the fightback.
Stuart Armstrong levelled five minutes later and Nadir Ciftci secured a memorable victory with the winner in the final minute.