Track Matte Not Working - only works in new composition

Hello, I am a beginner in AE and got stuck with using track matte.

There's this promotional video I've been making, and I wanted to put the Union Jack flag inside a text as the product is from England. Just a basic and simple image-under-text.

But whatever I try, the text would disappear when I would track matte the flag image with text. I didn't got the order confused and watched several tutorial videos and searched through the community in case I missed out something. But it wouldn't work out well like those instructions as seen in the screenshots below.

I also tried it on a dark background with text without color or strokes, but it just wouldn't work in the same project file I am working on.

But I got the order right and the function worked perfectly fine when I made a new composition with only two layers in it- image and text.  It just wouldn't work the same in the video I was working on. 

Perhaps, is it because of other layers I already made in the video? Isn't the track matte just about interaction between selected layers, with other layers having nothing to do with it? Could someone please explain what I possibly missed out? This should have been a simple task, but it's driving me crazy and track matte wouldn't work whether it's shape or text.

The image won't 'show through the selected layer like window'. Instead the selected layer I track matted would just become invisible with the image unchanged in its place. I am sorry for bothering with such basic function... Perhaps the tutorials I searched didn't explain the issue because it's something so simple, but I just cannot figure it out myself..