hmm.. is the its aren't showing in standard reports either that means some issue with the client-end not in your GA / GTM settings
Perhaps GoogleAnalyticsObject (or even GTM) isn't properly instantiating on some mobile device / browser combinations! Check in dve. console, whar’s GoogleAnalyticsObject object name (usually its “ga” or “__gaqtracker”) and if it’s instantiated properly? share site URL pls.?
also try contact +Google Tag Manager for WordPress
, if they've any official explanation to this ?