Sorry for not including it but here is the InputAction bound to it:
PlayerInputComponent->BindAction("Sprint", IE_Pressed, this, &ADynamixCPPCharacter::StartSprint);
I just tried to set it to Reliable, and it makes no difference.
The only other part pertaining to this function is in the .h file:
UFUNCTION(Unreliable, Server, WithValidation)
void StartSprint();
void StartSprint_Implementation();
bool StartSprint_Validate();
StartSprint_Validate jsut returns true
Thanks for your reply.