Hi there, Fero.....WELCOME TO GAMEBOOMERS!
Maybe you are starting too close to the seal. I might be crazy (some would confirm that
) but I think I started the letter opener insertion a bit up into the flap of the envelope and then brought it down into the seal, holding down the left mouse button. There may be more that one way to break the seal but there does seem to be just a tiny pixel location where the letter opener works effectively. You'll get it!!!
Best wishes, Witchen =O)