I use MMM 16 Premium but first here is an image from MMM MX Premium. Look at the red arrows on top left and right. That is called marker. We can move these two markers to left and right. Normally when we start Music maker it is set to 16 Bar. But when we record or add audio it must moves accordingly. In your case why it is not working strange.

Here is a screen shot from MMM 16 Premium

Usually a song could be of 100 -130 Bars. There is no need to fill the entire first track.

Try to import Demo songs of Maigx then look at the bar position. Export the song in wave or other format  and check the duration of exported item.

As you can read in the second screen shot we can increase the Bar by stretching the second bar to right or by clicking on the Minus sign. I prefer using Minus or Plus sign as per case. You should play a while to know the difference.

I hope this answer might help you.

nihon94