Territory losses military pressure, financial problems and poor management have emerged as main reasons behind militants low morale to fight.
"Now some militants are so disillusioned they are looking for any way they can to get out of the fighting," a report by the US military's think-tank, the Centre for Combating Terrorism (CTC) said.
"This was reflected on a wider level when the Islamic State issued a general amnesty for deserters at the beginning of October 2015. The personnel shortages were also evidenced by an Islamic State document that emerged last year," it said.
Amid financial hardship earlier this year, ISIS decided to reduce the salaries of all its militants by 50 percent. The decision was taken after the massive airstrikes launched by the US led international coalition hit its banks in Iraq.
The militants now get the equivalent of just 100 pounds a month because of "exceptional circumstances", according to a document released by Bayt al-Mal, the Treasury Ministry of ISIS.