The phrase "unsuccessful spell" means that someone's attempt to do magic or make something happen did not work as they had hoped or expected. Full definition
A trio of Bees Hall of Fame members, Jimmy Jay, Keith Millen, and Dai Hopkins, have also turned out for both sides while one man remembered for slightly less illustrious reasons, Murray Jones, had unsuccessful spells at both clubs in the early 1990s. (brentfordfc.com)
He's now at Sassuolo via unsuccessful spells with Pescara and Sporting CP. (squawka.com)
A former England captain and mutliple Premier League title winner, Adams suffered from unsuccessful spells in charge of Wycombe Wanderers and Portsmouth, leaving him virutally unemployable as a first team manager in his homeland. (caughtoffside.com)