Trimming of the roots will promote new root growth. Your plant could be to the point that there's so much old root in certain places that new ones have no where to grow in. It's not the old, thick roots that do most of the work of bringing nutrients into the plant, it's the new ones, so if your plant can't grow new ones... problems.
Pull it carefully, they tend to develop extensive root systems.
Other possibilities include Fe OD or a deficiency of some other nutrient, too little light, too much light etc. etc.
If the Eco complete isn't yet 2 years old and the roots are "OK", I'd back off on the root tabs... An alternative to tabs is to use a syringe, turkey baister or transfer pipet to inject a little of your liquid fert mix into the substrate near the sword. It's also cheaper than root tabs. Personally, I suspect that the plant may be getting a little too much fert loving.