For solvent-based products: Efflorescence present on the concrete surface. Solvent in the film has evaporated before the film had formed (when applied in direct sunlight and high temperatures.)
For water-based products: Too much bleed water present at time of application. Wait until all bleed water has dissipated. Poor adhesion to substrate will leave air voids between film and concrete. Make sure substrate is cleaned properly. Water is trapped between sealer applications (dew on surface).