I own Yamaha flute 281 and before that, I owned a Cosmos (it's a brand) flute. The Yamaha's list price is around $1000 these days and I purchased the Cosmos for around $300. So the Cosmos is a beginner model, Yamaha is an advanced student/beginner model.
At first, I had the same problem as you. I found that I could produce a much nicer tone on the beginner flute than my Yamaha flute (with plugs). It was easier to blow basically. But as I made progress on the flute, I found that the Yamaha model had a much better sound. So I guess the beginner flute can allure you into thinking that it has a nicer sound, but as you get used to your open-hole flute, you will find that it has the potential to produce a much nicer sound than your beginner flute.
If this still does not work, then there's probably technical issues with your open-hole flute and you can get it checked at your local flute technician.