my ds#2 is just 4mo & before he got sick (last week) he was doing close to 2 hours, but now 1.5 is his absolute max.
My thought is if she is happy being held, or "watching something" it may be that she doesn't like being left alone, my ds#1 was like that,. Maybe try changing the activity/ position every 5mins. I tend to move Liam from room to room, tummy time to propped up/ bouncer to flat on his back. I notice he gets very grumpy if he is on his back much during activity time & when he was in hospital & the same place for 3 days, I relised he was so much more interested in things & got "bored" quiet quickly, but as soon as someone came in the room, he was happy again.
I guess what I'm saying is she possibly needs more stimulation & change to keep her up longer.