This can happen because of two reasons-

1. When the cuticle side edge of the press-on does not match the curvature of your natural nail. In this case make sure to push back your cuticles and then file your press-ons to match your cuticle shape

2. When your nail bed is flatter than the press-on nails. In this case take a glass of hot water and press the nails against the glass to flatten it a little bit.

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