The 27-year-old underwent surgery in December after he sustained tendon damage to his ankle during Ulster's RaboDirect PRO12 match against Edinburgh on November 2.
Ferris had expected to be back in contention next month, but Ulster have confirmed his comeback has been delayed, and a statement on the club's official website read: "It was discovered that, although Stephen has made excellent progress in his recovery, some minor issues remain."
The statement continued: "As a result, the specialist medical-staff treating Stephen are considering several options to ensure that he makes a full and complete return to play.
"A decision on a course of action will be made in the coming weeks."
Ferris missed Ireland's autumn international campaign due to the injury and has been ruled out of the RBS 6 Nations.
It now looks likely that the back-row forward will be unavailable for Ulster's Heineken Cup quarter-final against Saracens at Twickenham on April 6, with his place on the Lions tour also in doubt.
Ferris missed the entire Six Nations tournament in 2011 through injury and saw his 2009 Lions tour of South Africa cut short due to a knee problem.
- Sports & Recreation