thank you akif for the kind words but I don't consider my self expert.
Here are few things I know which is required for eco idle to work,
- driver seat belt must be buckled.
- The battery is "sufficiently" charged and powerful, and also the charging mechanism (generator) is also working.
I guess there can be two possible problems , check your battery water level and also get its 'gravity' checked from some battery shop. 2ndly , yes this can be a system problem, for that you can go to a workshop which has computer checking equipment they can check and see if system gives any error. One possibility is may be the corresponding fuse has burned and needs to replace.
At the last I would say you are getting very good mileage , and eco-idle contrbutes "very" less to the fuel economy. i mostly turn it off, its no use on small roads, or in rush hour traffic of city. its only benefit is when you are travalling on roads with lots of traffic signals but moderate- to low traffic. even then it only save few mL of petrol. In summers with AC , everyone turn it off so that AC keeps running on stops. So in my opinion its not that big a deal. 
