Have you tried overflow?
By strengthening your patients trunk or his/her other leg you will certainly get an amount of overflow and you can use this to facilitate more strength.
Look carefully at what the reactions are, think up new challanges and chances are the strength will improve.
Think muscle chains and work out which chains can be used ( even throught the upper limbs) to facilitate hip flexion.
Once you have this going and the flexors react, then make sure your patient repeats and gets a chance to tire his/her muscles... go for the endurance...
Good luck Esther