The biggest culprits are: still using SKAGs (single keyword ad groups) when this strategy is dead due to close match variants, conversion tracking not being set up properly (so your campaigns look like they are doing really good... when they actually are not), and relying heavily on broad match keywords with no guardrails (AKA fully trusting the machine). Those are just a few examples, but there are many things that can be hindering the performance of your campaigns!