Dog won't walk with me, but will with other walkers?

Hello!

I was recently hired to walk a dog for a few days. She's a sweet older girl, but she tends to stop walking after only ~15 minutes. I asked if this was her typical behavior. She seems to have no problems walking with other walkers. How could I get her to walk with me? I try to make sure she gets her thirty minutes since that's what the clients pay for. But she doesn't seem interested in walking and will pull towards the entrance of the apt building after a few minutes.

Any tips would be appreciated! Thanks in advance