The state-of-the-art Sentinel reconnaissance aircraft was forced to stop in Senegal with an unspecified technical problem, the Ministry of Defence said.
A spokesman said they hoped to have it repaired and on its way to join the international rescue effort "sooner rather than later".
No specific deployment time had been promised, he said, adding: "We will get there as soon as we can."
The aircraft of 5 (Army Cooperation) Squadron left RAF Waddington in Lincolnshire on Sunday and was due to fly to Accra, Ghana. The aircraft has a crew of five and is capable of flying for significant periods of time at high altitude.
It is supposed to be joining US aircraft in attempting to locate the girls, who were seized by Islamist extremists from a school dormitory in the north-east of Nigeria on April 15.