You might want to spray a little chelated iron on the leaves. The plant might be able to take it up from foliar feeding even though the roots are unable to.
A wrong soil pH can interfere with iron absorption. Iron needs some acidity to be available (pH 5 to 6.5). As Paul said, excess lime or any other cause of high pH could have made iron unavailable in the soil.
Can you check the pH of the soil in the pot, several inches down?
Some other mineral, if present in excess, can also interfere with iron's availability. Was it Phosphorus?
("At alkaline pH values, greater than pH 7.5 for example, phosphate ions tend to react quickly with calcium (Ca) and magnesium (Mg) to form less soluble compounds. At acidic pH values, phosphate ions react with aluminum (Al) and iron (Fe) to again form less soluble compounds.")
http://www.nutrientstewardship...
I was never able to distinguish between iron deficiency and
magnesium deficiency, but I think Mg is easy to absorb compared to iron. Still, spraying a few leaves with Epsom salts is easy and rules out any problem related to roots.
Wikipedia says that a
manganese deficiency is easy to confuse with iron deficiency. I don't know about that.
I forget: have you flushed the pot or assured that salinity is no problem?
Are you able to, or have you already, looked at the roots to see if it is pot-bound or the mix has broken down and become compacted? It's five years old, has it been re-potted?
