No peak/missing peaks in Head space GC injections using 7697A sampler

Recently, we have an issue with Head space GC injections (usnig 7697A sampler), when running a sequence of residual solvents method,

we had a sequence of about 50 injections, and 2 injections produced a flat chromatogram with No peak, the diluent peak that supposed to be a very high peak, is barely seen and 

of course the other analytes (residual solvent) are completely absent (No peaks).
this happened when we used screw cap and crimp cap vials (20mL vial).and it happened in more that one GC system equipped with this type of sampler, 
we should also state that we use Empower 3 to run the system.
What should we check ? 
Which 7697A parameters should be altered or checked ?
Can this be due to 7697A error and somethig need to be fixed ? (Hardware)
Can this be due to the vials used ? does it matter if screw or crimp vials used ?
Is this phenomenon known to you or other users ?
Your help is highly appreciated .
Many thanlks,
Maher K.
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