Hello Anne,
welcome at Stridaforum 
Well…it is like Bill said - the barrel adjusters are nowadays intended to adjust the outer pads, you can find this info also in the CD manual.
The document “Disc Brake.pdf” refers to your, newer version of the brake calipers,
“Colourful Disc brake.pdf” is in fact the older version which is shown at the user manual.
In your case - I believe - may have happened something with the actuation mechanism inside of the caliper. As this could generate a safety issue I’d recommend to replace the caliper.
You wrote that your Strida is young - did you buy at a local store?