I have created a poi file (ov2) and 2 icons (bmp and X2.bmp) but the icon does not get displayed on the map. What is going wrong and is there a fix? Are there any secret constraints, like using/not using capital letters?

The icons are exact copies from another poi set where they work OK.
If I search for nearby pois then the icon shows on the dropdown list but does not show on the map after choosing it. Obviously I have ticked the box asking for that poi to be displayed.

Machine is a TomTom One.

What happens when you do a find on those POI's ?

I suspect you have mixed up your Latitudes and Longitudes !
TomTom POI files use the opposite convention to normal !

Thanks for the suggestion but I haven't confused the order. The poi finds the correct point on the map. I can search for a poi (in this poi set) near a place and it offers me a list of several, sorted by distance. As I walk through them it shows details and a map with the icon. If I choose one of them to display on the main map it highlights the correct position but the icon is not displayed.

What happens if you delete the *.bmp icon.
Do you get a red square ?
If so does it appear everywhere your *.bmp icon did ?
Do you get the red square on the map ?

I haven't tried seeing what happens if there isn't a valid icon because it suddenly started working for no apparent reason. I'll leave it alone unless it misbehaves, perhaps when I add another poi file.