Ti:sapphire laser does not mode lock at 990nm

We have a tunable Ti:sapphire laser that does not mode lock at 99nm but it works fine at any other wavelength. I have ruled out the humidity as a cause.
The representative suggested that this is probably due to drifting away from calibration. Which I am not sure what exactly mean.
Can someone explain to me why would a laser have a hard time mode locking?

Thanks in advance.