Validity of Agreement to Sell

Sir/s, My mother purchased property in Delhi(Tenement A type Flats) in 2005 via registered GPA and Registered and stamped Agreement to Sell on the same day of May/2005 My mother applied for freehold conversion to Land and Development Office Nirman Bhawan Delhi. L&DO is saying that the "Agreement to Sell" is not valid because it is purchaser to purchaser. My mother got the Registered / irrevocable GPA from Mrs Sita who was the GPA holder and Agreement to sell from Mr. Ram (Husband of Mrs Sita), who was the agreement to sell holder. So the same chain was copied in my mother's name i.e. Mrs Sita did the irrevocable / registered GPA to my mother and Mr. Ram gave the agreement to sell with full consideration amount / receipt / possesion etc to my mother in 2005 on same day both GPA and Agreement to sell was registered in sub-registrar office delhi and full stamp duty was paid on registered agreement to sell. The original Lesse was Mr "Kumar", who executed Agreement to sell duly registered and stamped to Mrs Pyari and registered / irrevocable GPA to SON of Mrs. Pyari i.e. Mr. Prakash.... Mrs Pyari got the property mutated in her name from L&DO somewhere in 90s. Somewhere in the 90s only they sold the property to Mrs Sita and Mr. Ram. Where Mrs Pyari executed registered and stamped Agreement to sell to Mr Ram and Mr. Prakash executed registered GPA to Mrs. Sita. Post this the above, the same chain was copied to my mother as stated above. Please advice.