Does it not turn or when released it does not lock the pin in the blade hole ?
Sounds like you have sawdust/metal in the mechanism.
If you remove the guide and try standing the front in a pot of white spirit or similar then it may allow it to flush out what ever is jamming it up.
I should think most of these saws use the same principle for the blade holding so a manual from say Ferm or Makita may give an exploded diagram that Aldi don't.
I saw these in lidl recently, are they any good on thicker metal or just sheet?
I wouldn't class a reciprocating saw as an 'accurate' tool, in the US they call them demolition saws which seems right to me.
I do have a TCT masonry blade for mine which is handy when cutting window appetures and doorways through existings walls but for everyday cutting of most materials there are faster and more accurate tools.