Try to seed the random engine. Time is a good choice. Read this to get familiar to another ways.
std::default_random_engine generator( (unsigned int)time(0) );
or you can use std::random_device
which tries to produce non-deterministic random numbers
std::default_random_engine generator( std::random_device{}() );
std::random_device is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.
Note that std::random_device may be implemented in terms of a pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation.
It uses a hardware random generator device or generates a pseudo random number. Useful to seed.