I had a local tire garage fail to put the stem caps on tight. Drove away on my brand new tires, kept wondering why they were quickly becoming all mushy. Either one of the grease monkey deliberately sabotaged them to leak, or they were too incompetent to have been hired in the first place.
The caps are not intended to retain any pressure, they are primarily there to keep debris out of the stem. If the presence of the screw cap is preventing any significant air loss it means the valve core is loose or the seal is damaged and leaking.