Why doesn't desktop unlock screen accept my password?

To unlock a my computer after waking it from standby or screen saver I sometimes have to enter my password many times before it's accepted. What's wrong with it?

UPDATE:

NOT a wireless keyboard - a plain wired Dell RT7D50 USB keyboard and the laptop's built-in keyboard, same behaviour.

All the dots do appear.

Sometimes switching language to a different and back to English seems to help.

There are no problems with keyboards in any other case - only when entering the password in wakeup unlock dialogue. For example when I install applications or reconfigure something and the system asks to enter the password - it is always accepted immediately.