This is most likely due to the route image (RI) still being low. This needs time to build up, and when it will be, your flights should be full, don't worry.
LH is the extreme example - RI is much slower to grow on those, and the operational costs of LH planes are extremely high. So you could lose money for a whole year before making money. But once those are stable, they make an awful lot of money.
Nonetheless, for MH, it's usually useless to invest heavily in route marketing. Newspapers or billboards for a month or so should be enough.