None returns when running gemini model in async mode

[gyi123]

I am running a few dozen cases using gemini mode for evaluation, I run each case with each metric successfully. However when I run all of them in async mode together using the code below:
result: EvaluationResult = evaluate( test_cases=test_cases, metrics=[claim_metric, answer_relevancy, faithfulness, contextual_relevancy], async_config=AsyncConfig( throttle_value=2, max_concurrent=4 ) )
I sometimes get none verdicts or claims or truths. When I debug debug, sometimes the code return None for res.
res, cost = await self.model.a_generate( prompt, schema=ContextualRelevancyVerdicts )
Anyone has the same experience? I did play with throttle_value or max_concurrent, it helps, but does not completely eradict the issue.
Any suggestions?