There's really not that much more to paladin tanking than doing what you said - Halone Combo on the primary targets, Flash for the secondaries. As you get more experienced, you start spreading your halones out amongst multiple targets when you know how much threat you have (eg, tanking 2 targets you might throw the Fast Blade and the rage of halone on the primary target, but tab to the second mid-combo to hit it with the Savage blade). It's all about monitoring your threat and knowing which targets need more.
AOE tanking is a lot like "spinning plates". You have multiple threat bars to monitor and you have to ensure you're at the top of each mob's bar set. What rotation you do to keep threat varies hugely depending on the party. Sometimes you get a party with super geared single-target dps who go nuts on a main target, and you barely have time for more than one flash before you're forced to spam pure halone combos on the main enemy or risk losing threat. Sometimes you get into AOE happy parties and you need to throw in four times as many Flashes as normal and all your Halone/Savages get spread evenly across an entire pack. It's not that hard to learn, it's just something that varies.
End of the day, if every mob is aggroed on you and you're not completely dying, you're doing fine. Every tank loses the occasional mob's threat (that's why we have provoke).
I wonder if your FC is not talking about your skill use, but your -pacing-. The tank is responsible for the speed and smoothness of the run. I 100% agree with everyone above that any FC that says they'll ragequit on you isnt worth staying in, but maybe they mean the pace of your tanking. A lot of players get very itchy and impatient if a tank stands there for a few seconds each pull, marks everything, checks people are ready, then stops between packs and such. Sure, it's safe, but there's a middle ground to be had where ideally you should be running every single dungeon at the maximum acceptable pace for the capabilities of the group.
It doesnt mean you need to speed run every instance and aoe 5 packs at a time each instance. It just means you should be running to pull the next pack when the final mob in your current pack still has a few hp left. You should be marking targets as your Shield lobs fly out rather than pausing to mark them first. You should be going as fast as possible if the group allows it and it's safe to. Obviously on new instances where you're learning pulls then take your time, but once a dungeon is "learned" then people expect a certain pace and fluidity to the run, especially people who arent exactly close friends and understanding.
It may be that your pace is fine, but I find the most common complaint about tanks (outside of bad tanks who literally can't hold more than one mob on them at once) is that they go too slowly. It doesnt mean speed run everything, it just means keep a good pace and try not to be the one that's holding up people. And that comes with confidance and experience.