Stamped concrete is typically sealed with an acrylic concrete sealer and when water sits on top of the sealer, it makes the stamped concrete slippery when wet. The best way to reduce the slipperiness of stamped concrete is with a non-slip additive.
Non-slip additives are typically polymer-type additives that resemble a grain of sugar (fine) or sea salt (coarse). It is mixed directly into the sealer and applied to the stamped concrete. If your concrete is already sealed, an additional coat will be required to apply the non-slip additive.
Most non-slip additives are clear and barely noticeable when applied to the concrete. If over applied, the gloss level in the sealer can be slightly reduced. Most manufacturers carry their own line of additives, but other top rated additives can be found here: