Like other offers in the game, Training ads availability can fluctuate depending on how many the offer providers can give to your profile at that moment. It depends on location (country), profile, language, platform ...

It may happen that on specific moments you don't see any, but try other platforms, as some offers are only for one of them: if you have an iPhone, try Android or the web browser version, for example, as providers may have offers for you but not at the platform you're using.