Did you do the trimming? Notice something different after trimming the tops?
My Aromatica grew think from the bottom. That is after I started dosing Calcium and Magnesium in my 100% RO planted tank.
If the trim didn't do anything with regards to thickness you should look for deficiencies.
I've noticed other plants starting to thicken after dosing the aforementioned minerals so I think it's safe to assume you might need to check your water and add more of what's missing