The waypoint must have a (T)urnpoint attribute. Such as:

5,39:02.138N,104:50.629W,2041M,TAL,Aardvark,

The "TAL" above mean it's a Turnpoint, Airport and Landpoint.

For contests, it makes sense to only have real TPs have the T
attribute, but for general flying around, all should have the T
attribute set so they appear in the task menu.

So, take a look at your DAT file to verify this.

-Tom