My guess is that the physics simply don't work out for this application.
The first step of your desired process is to extract the flavours from the herb, spice, etc into the surrounding water. This process will take time, even when you finely chop them. It's possible that the steaming is complete before this process has sufficiently progressed.
The next step is vaporizing the dissolved flavours into steam. Again, in the beginning you will produce pure water vapor and only over time as step 1 is taking place will you slowly get some flavors into the steam. It's possible that your flavors will be too heavily dilluted even at their peak.
Step three would be to attach those flavours to the target vegetable. Here I must point out that you're never going to get all the flavor to stick to the veggies and the water vapour to pass on. They will, if anything, stick to the vegetables in the same proportions as they exist in the steam.
You could try to steam them over a strong broth instead of water and see how well it takes on the broth taste. If it doesn't then step 3 doesn't work and you can forget about it. If it does then your problem lies in steps one or two. In that case I'd suggest that you first dissolve the desired flavor into water in a strong enough concentration and then use that water to steam the veggies.
Alternatively you could try and place the flavor source with the vegetables to see if the impart more taste through direct contact.
Finally you could try to produce an essence/reduction/sauce of the desired flavor and apply it to normally steamed vegetables.