Fix bug: everything frozen after second key pressed -- 2

I have a bug with my application. This application removes letters in several words to help user memorize texts. In order to help learning lessons, the program displays any text file sentence per sentence, and only shows start letters for a few words. The user must type remaining letters. With the new version, when I open a file, I can type a first letter, but when I want to type a second one, it doesn't appear on the screen. I want this fixed within 24 hours I won't be able to tchat about it. I will just access my mails. 1) it works all right on another computer 2) when I switch to another window (alt + tab) I can type another letter, but I have to do it again if I want to type another letter, and so on.