Problem with getting available fields (fieldsMetadata)

When showing service configuration page, using postMessage we get fieldMetadataEndpoint (amsAction:'metadata'), and from endpoint received in response using postMessage again we try to get available fields (amsAction: 'product).

Sometimes that second request is successful, but not always (cca 40% attempts failed). Is there some limitation (frequency, ...) for invoking postMessage, or some other reason why every attempt is not successful?

Thanks.