Automatic change of the numerical screen lock, why?

my daughter uses the four-digit number as a screen lock of her phone (Xiaomi Redmi 3S) but one day the phone wrote "incorrect". She did not change it so she can not forget it. It has happened three times yet. On two occasions he unblocked it through the GMail account associated with his phone, but in the latter it is not allowed to do so.
Questions:
1) Is there any way to unlock again on behalf of GMail?
2) What do we avoid doing to avoid self-blocking in the future?