I don't know if the selector in the Focus RS MTX75 box is the same, but they suffer from breaking rollpins in the selector shaft. The pin holds the selector forks to the centre shaft, and fractures over time.
As the pin weakens and fractures, gear selection gets progressively more difficult, until it eventually breaks and leaves you with no gears.
It's an easy and cheap replacement, just a new rollpin that costs a few cents/pennies