I looked for a long time for a vacuum machine, but could not find one, in France.

Then, a very simple model appeared on the market, and I bought that one (for about 50 Euros IIRC)
....
And 3 weeks later I saw the model I was really looking for ( more robust, more powerfull, larger bags, but much more expensive). <img src="/images/graemlins/crazy.gif" alt="" />

So, presently, I only have the cheap model. I too have the "loss of vacuum pb" you describe, either using the recommended bags or cheaper bags.
I don't know if air is coming in due to bad sealing or something else (poor quality of the bags, damaged bags,.... ?). <img src="/images/graemlins/confused.gif" alt="" />

Maybe I will give this one next Xmas <img src="/images/graemlins/grin.gif" alt="" /> and buy a superior model (9 times more expensive ...) <img src="/images/graemlins/cool.gif" alt="" />