This is usually because the plug does not have protection class IP44 (e.g. IP20 angled plug).
brennenstuhl® socket strips, cable reels, power distributors, extension cables or adapters for outdoor use have a protection class of at least IP44. Among other things, this means "splash-proof".
To ensure this, the sockets or couplings are fitted with a rubber lip. This rubber lip makes the socket outlet or coupling slightly deeper than those for indoor use (protection class IP20).
IMPORTANT: If the plug connection is really to be splash-proof, the plug must also have protection class IP44. Also make sure that you insert the plug fully and with enough force until the rubber lip is pushed over the edge of the IP44 plug.