Perhaps the Space Traders Flight Training Manual was to blame for the difficulty.

http://www.elitehomepage.org/manual.htm#A13

The manual was bundled with the cassette or disc, and it states in the 'Docking Procedure' section:

Approach the final moments of docking at DEAD SLOW SPEED

However this is dead wrong! If you put some speed on before entering the 'letterbox' you're actually far less likely to crash, unrealistic though this may be. It's perfectly possible to dock successfully at full speed. Try it out here:

https://www.bbcmicro.co.uk/game.php?id=366

Oddly enough, the docking computers are also capable of crashing your ship.