During the week, there is a direct train Brussels-Maastricht - stoping in Liège too, but you don't have to change the train in Liège, easier.
If you cannot get this one, simply do Brussels-Liège, then Liège-Maastricht.
tip : Liège is the name of the city in French. You will have "Luik" in dutch.
check
NMBS - SNCB for more info