Issue with Wheelchair Service on Turkish Airlines Ankara-Dusseldorf Flight

I bought a ticket for my mother on Turkish Airlines for the Ankara to Dusseldorf flight on October 25th at 14:30. The ticket was quite expensive, but we chose THY for my mother's comfort. We made an online reservation for a wheelchair and also paid for luggage, meals, and seat selection. However, during check-in at Ankara Esenboga Airport, they said they couldn't provide the wheelchair and that approval was needed. Another family experiencing the same issue also didn't receive approval for the wheelchair. My mother is 78 years old, has had knee surgery, and has difficulty walking. Unfortunately, she had to walk with her hand luggage. Previously, my mother traveled with different airlines and never faced such an issue. We trusted Turkish Airlines and spent extra money, but it was a complete disappointment. If we had known, we would have chosen cheaper airlines. They could have informed us if the quota was full or resolved such an issue, being a major airline company.