Problems with using the Master Public Key - SOLVED

Hello All,

I wanted to create unsigned transactions with an offline computer. My first step was to set up a watching-only computer.
I tried this:
- saving the master public key I get from Wallet->MasterPublicKey to a text file: No problem
- installing electrum on a new computer using the "import master public key" option (or something similar) during the installation routine

Installation no problem, but ...
Now, I can find some of the keys I have in my (old) electrum wallet, but not all of them. I do not understand why, because I cannot remember to have willingly created any new addresses. I do not understand at all. I am pretty certain that I have not imported any keys manually.

So does Masterpublickey only work with the old addresses generated during the installation of Electrum? Where do all these new addresses come from?
Shouldnt they be included in the Master Public Key (which is something like a Public Seed if I understand correctly)

Is this a normal problem or have I done something wrong?

(P.S. I am now doing a work-around by using the "send-from" option sending from one of my addresses not included in the Master Public Seed to an address included in the Master Public Seed).