Emulsion and screen exposure.

Emulsion is the photosensitive coating spread over the mesh, and exposure is the UV step that crosslinks it everywhere light lands, so the shielded image area washes out and leaves an open stencil. Underexposure is the failure that shows up most often, and the tell is physical: during washout the squeegee side feels slimy, or emulsion color lifts onto your finger. Overexposure has the opposite signature, a stencil that will not open up in washout no matter how long you spray it. Timing exposure in seconds is the quiet trap, since a lamp emits less usable UV as it ages, and the setting that worked last quarter is underexposing today until the stencil breaks down mid-run and stops the press.