Often my experience on Zwift:
Although I produce more w/kg than riders ahead I find it nearly impossible to catch up to them.
This is a sustained effort (3-5 minutes) at much higher w/kg power (4.1 w/kg) than those ahead (2.6 w/kg). At the very least I should see the lagging time decrease.
Anyone has an idea why this is happening?
Welcome to the forum.
If it is on a flat route then w/kg is irrelevant you should look at raw power.
If they are in a group then they will draft each other and you will have to work a LOT harder to catch.
Thank you.
What is raw power?
Franck
Pure Watt numbers, rather than W/Kg.
What bike in game are you on?
Be sure you are on a S-Works Venge with the DTSwiss Disc wheels for flat routes or the Zwift Concept bike.
The rest is down to the average power in watts - on flat routes the big riders have the advantage because they thunder along at 280w.
If you are lighter, ride on Ventop or ADZ where those big guys can’t cruise along - they have to work much harder. 
If you have steering, steer all the way over to the edge of the lane you are in once you’ve passed someone, this can help prevent them drafting you.
Thanks, I appreciate it!
Thank you!
you need to put out >30% power to catch someone who is drafting if you are a solo rider.
frame / wheel choice also affects speed (but not that much).
most likely though - if it is during a free-ride - they could be doing a private group event with “keep everyone together” switched on, in which case their w/kg will not correspond to their speed at all (could be with stronger riders who are pulling them along - the feature will average the speed of the group)
Very helpful, thanks Ben!
Franck
Welcome to Zwift’s wacky physics. Yeah, it does not make sense.
That is just pure physics. Catching a group of riders IRL also take a substantial amount of power especially is they are going at race pace.
Level 42 for those wheels. I don’t think these are questions a level 42 rider would ask 
Unfortunately, it’s not that simple Gerry. Sometimes you have IRL strong tailwind and can use it to break away. Then the chasers have a hard time and are not faster than a good time trialist.
We are talking about catching a group of riders ahead.
No. It is not pure physics. It is clown physics. The OP is producing way more power than the riders in the group ahead but still not catching up. That ain’t real life or anywhere close to it.
I think this is due to limitations in most trainers. Ideally when you catch the draft, the trainer would reduce resistance. But what to do when the riders are on flat ground and trainer resistance is already at its lowest level? The solution appears to be to make the whole group go faster. The side affect is anyone falling off the back is screwed.
There is a lot of information we don’t have. My assumption is it was a flat route and the group in front was big and had some bigger users.
Zwift physics is very close to IRL physics.
Anyone can ride with a Robopacer and see what the OP is saying is true.
Zwift is very close to IRL physics? Really? I am really wondering whether you have ridden a bike outside with a group.
So there are headwinds, crosswinds and tailwinds. Very nice. I love the zwift physics without wind. 
So lets say the OP is 60kg @ 4.1w/kg = 246w and the rider ahead was 100kg @2.6kg = 260W so the OP would not be able to catch if that was a group.
We don’t have a lot of information to go on.
I have done my share of road racing to know if you fall off the back at race pace there is almost no way to catch back up to the bunch.