On Strida 1 & 2 each wheel has the same plastic male clip (like a thin mushroom) , but in addition the rear wheel has another part attached to it … to form the female keeper fro the front ‘mushroom’ (sorry for multiple metaphors!!)… it sound like this ‘female’ part is missing.
These were easy to damage and break, and then if not noticed they can be lost. Mark sanders’ used to keep a stock but these are now all gone. Ming cycles have the tool , but unles many are ordered it would be difficult to get them to make only a few. I suggest you find a piece of rubbery tube about same size as the plastic male parts and fix this to the back (say with a nylon tiewrap) so that it is a slight snap/grip fit with the front part … to keep the wheels aligned and together.
If this doesn’t make any sense please see the Mk1 (also used for Mk2) manual which is available on Strida EU website eg …
strida.nl/english/service.ph … ge=manuals
or direct to PDF…
strida.nl/english/images/Strida_1_Manual.pdf
internet is great … its all there if you look hard enough 