The symptoms sound/look like overwatering!
Maybe the soil is too dense and doesn't dry out fast enough?

I have one of these plants and I hardly ever water it... in fact, the only thing that reminds me to water it is when it starts wilting!

I suspect over watering too when I repotted it, but other houseplants are in same soil and conditions. Should I leave it till the soil is very very dry. Any chance it could recover. When I repotted, I removed the rotten bulbs till there was just the stem end. Will it form another bulb and roots. Its been more than 6 months now. No new growth. The plant looks the same as when I repotted it. What is normally the sign of underwatering for this plant.

Different plants have different soil and water requirements. Just because a certain type of soil is good for some plants, doesn't mean it will suit all plants, unfortunately.

I allow my plant to get very very dry between waterings, and it's looking nice.
I think the plant could recover and start growing again - ZZs are tough! - but I also think you should take the plant out of the pot and see what the soil is like. If it's wet or compacted, try mixing it 1:1 with orchid bark chips before repotting the plant. ZZs like good drainage.
If there's a lot of extra soil in there with no roots in it, you should probably use a smaller pot.

Just looking at the picture again, I think your plant may be planted too deeply. Usually, the bulbs are not completely buried. You should be able to see most of them above the soil. Burying them could cause them to rot.

When I have underwatered my plant, the stems wilt and start to look a little bit shrivelled. Sometimes the lower leaves turn brown.

I really hope you can figure out what's wrong! At least the little leaves down the bottom look good!

Thanks for your response and advice. I live in a place where I wont find bark. Could I try a little sand mixed with the same soil. I know the bulbs/roots are completely buried, will check it today.

No sweat! I'm glad to help!

You can usually buy orchid bark mix at nurseries and hardware stores.
Coir chips or perlite would also be fine, if you can get hold of them. Again, look for them at nurseries and hardware stores.
I haven't had good experiences with sand added to potting mix. It usually just seems to make it really heavy.

No sand pls. it will likely make things worse. I'd look for pumice or perlite & add 50% of that.

These plants behave like succulents & don't want frequent watering. I water mine maybe every 10 days to 2 weeks, no more.

Keep in mind that if the conditions are right, and the plant does not seem to be growing, it may be focusing on roots.

With a zz plant, especially one that had or has root rot, it is best to water very minimally. I almost killed mine. Now that I remember to neglect it, it does nothing at all. But that is better than a slow (or fast) decline.

The ZZ Plant is really quite mystery, what works for one doesn't for the other. One of mine is constantly thirsty and I water it every week (more so in Summer, less in Winter).

However bottom line if the tubers are rotten then its been over-watered or exposed to frost, nothing else does that sort of damage.

If you manage to salvage it, or get a replacement, don't worry about the lack of growth its another quirk! You will find when things heat up and there is a lot more light around things burst into life.

That's right. These plants spend most of their time not growing. Then occasionally they put up new canes.

And feathered_horse makes a good point. Not all plants, even if they're the same kind, have exactly the same needs. How much water a plant wants, for example, can depend on so many different things. I know that if I moved my ZZ to a spot in front of a sunny window, I would have to water more.