Google Publisher Tag - not working as implemented

Question

I've implemented the necessary code to show and ad on one of my pages of my website but I've noticed the ad fires sometimes and the other times it doesnot fire? I'm new to GPT so I was wondering why this was and if i can get it to fire everytimes

 googletag.cmd.push(function () {
 advert.slot = googletag.defineSlot(advert.id, [advert.width, 
 advert.height], advert.elementId);
 advert.slot.addService(googletag.pubads());

 googletag.enableServices();
                    
 googletag.display(advert.elementId);

  Logger.debug(advert);