I have the same problem as this one described here: Prebid.js + GAM In short - the native ad from prebid is not always rendered.
I see in GAM that adapter returns all required fields and there is an active line item with settings adjusted to run such creative. But somehow it is not being selected by GAM (the ad slot stays empty). If we rerun the process as a simulation we can sometimes find the same ad returned by prebid and assigned to particular line item in GAM. I do not know why sometimes the same ad is passed to the line item, ad sometimes - it is not. Please let me know if you have any experience and ideas why it works like that.
I have checked the line items setting, prebid price granularity, native format and styles settings and everything seems ok. The most problematic thing is that the same ad slot sometimes displays particular ad and sometimes the same ad winning prebid auction is not displayed. I do not know why it works like that.