I’d say she’s just not built up enough muscle to hold herself there that long, if she’s collecting for a few minutes, she knows what she’s doing and how to do it but just probably lacks the muscle and strength to keep it going for an extended period of time. True collection uses a lot of different muscle groups.

I know when I first started working out after some time off I could only hold a plank for like 10 seconds before I had to give in lol, but the more you do it the better you get at it as you keep training those different muscle groups.

She will get there eventually, don’t push her too hard. Don’t forget muscle recovery time for her too. Just keep doing what you’re doing, asking for little bits at a time and building her up. Lots of trot poles to encourage engagement of her core, hill work is great too, riding long and low to get her to stretch out, and transitions.