Today, I received an envelope missing the stamps. They had been on when the envelope went through the Canadian postal system (there are postmarks visible), but the stamps have come off between then and delivery to me.
It isn't the first time stamps have fallen off envelopes to me. These two were sent from France, again the stamp cancellation marks are visible.
I think the problem has been that the envelope paper (maybe from a magazine) is too glossy/shiny for the stamps to stay put. I had this problem on one envelope I made, and it was returned to me due to "insufficient postage". I contacted Royal Mail, and they suggested affixing the stamp to a label/something not shiny and then sticking this to the envelope.
These are two envelopes I have made. To seal, I use double-sided sticky tape though I am not convinced that all these tapes I have are of great quality. I'm not convinced of the stickiness of some cellotapes, they are easily peel-able, not ideal when you want something to be stuck permanently.