Your problem sounds like a classic case of not watching your caps lock and num lock indicators when you enter a password. The passwords in Linux are case-sensitive, that is, it distinguishes an a from an A. Since most password entry screens don't actually show you what you are typing, you, personally, have to ensure that the caps lock and num lock keys are in a known configuration and that you are not using the <Tab>, <Esc>, Windows key, Menu key, Print Screen key, Scroll Lock key, Pause/Break key, a cursor movement key, one of the function keys (F1 through F12 at the top of the keyboard), the backspace key, INS key, DEL key, Page Up or Page Down key as part of your password, as the actual keycode of these keys can vary depending on the program environment you are running.
LMDE Mate 64-bit, LM17 Mate 64-bit
Debian Mate 64-bit, Xubuntu 14.10 64-bit, Ubuntu-Mate 14.04 64-bit, Antergos Xfce 64-bit, PCLinuxOS Mate 64-bit