HMC833 Lock Issue

Hello

We are using the HMC833 PLL in my custom HW design. We have produced a number of HW board with the HMC833 PLL and We observe that:

1. In a number of boards the HMC833 does not lock but on some of them it does lock. The HW design is exactly the same on all boards

2. In some boards where the HMC833 does not lock at board init process, it locks after some time, once the boards is heated a bit

What can be the source for the observed issue?