You should always pre-wash new garments to ensure maximum glue absorption. Use the following simple water test will indicate whether fabric will absorb the hotfix glue.
Hold the garment horizontally and place a drop of water onto the fabric. If the water soaks into the fabric within 2-3 seconds, it is ready for hotfixing. If the water sits on the surface of the fabric or takes a long time to soak in, there is insufficient absorbency for a successful result. Re-wash the garment and retest when dry.
See our other guides on; Create your own Crystal T-shirt Transfer, Bejeweler Tips & Hits, Using a Hotfix transfer.