Hi Silentsoul, welcome to Pixies!
Firstly I must congratulate you two on the level of communication you clearly have - it's essential when experimenting with anal!!
As to your problem, I'd suggest that you do your play after she's just taken a shit, so that she's all clean there anyway, and she's empty so she definitely won't need to go.
After that, it's probably just a mental thing. She needs to relax her whole body, especially her anus and especially her mind!

Good luck, silentsoul, let us know how it goes!
CapserTG

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You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

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https://imgur.com/a/80hiRdm