Gaining experience for infantry is tough. You have to fight pitched battles with other light infantry, which may result in heavy losses even if it comes with chevrons. Personally I have only played as Carthage. I have fought A LOT of battles so far, the vast majority with Rebel factions and slave revolts in my home provinces (which while I have them in abundance, there are far less rebel settlements and armies near and around the Roman provinces). Currently I have two Generals with three Gold Chevrons, and several with silver. And that is simply because I use them heavily, only using other troops as backups in case my heavy cavalry struggles. I have gotten virtually no experience for my infantry and even other skirmisher cavalry.

However with Rome, you have less chances of facing opposing enemy infantry. Let alone be able to engage them h2h with your infantry and kill the hundreds of soldiers needed to gain even a single chevron. Just keep at it and retrain your units of infantry when ever possible, don't combine them. You have more of a chance to keep and even gain chevrons when you retrain, so make sure you don't get your even slightly experienced units wipped out in a battle.