Perhaps there might have been some defect in the adhesive gum?

Jack cut out a patch of balloon silk large enough to fit the hole and spread it with the adhesive gum solution.

Seated in his private office he wetted the flap of the stolen letter, and waited patiently till the adhesive gum could be loosened.

It had been closed as usual by means of adhesive gum, which had been made to give way by the application of steam.