Horizontal OpenLOCK ports print with small supports to support the overhangs in the port mouth. You need to remove these supports post-printing before you can insert the OpenLOCK clips.
We recommend using some needle-nosed pliers for this task. You can use the pliers to grip and twist the two supports in each port, breaking them and pulling them out in one fluid motion. This will often leave small nodules on the inside of the port. If these nodules are too large, you may want to go in with a hobby knife to trim them out. Be careful not to break the dividers between the OpenLOCK ports, as they provide pressure to make the clips hold.
With the supports removed, you are all ready to start assembling your epic dungeons.