Raise the temps (warm side 95-100F, cool side 75-80F). When I first switch sand boas to f/t I wait for them to sit with their head at the surface of the substrate and dangle the mouse to provoke a feeding response. Once it has constricted the prey item you can either leave it in the enclosure or move it to another container...

Best of luck,

-Chris
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Christopher E. Smith
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Captive Bred Herps