Do they look like this?
The underside of the burners corrode.
You can try to vacuum out any stuff in there. Remove them and shake, poke and prod/inspect.
I replaced a couple of mine last week. A friend had a dead stove and I just grabbed the ones from his. The garbage dumps etc are also full of them. I found that they varied in two main ways.
The support bracket on the left may be in a different orientation (was no problem in my case I just screwed them down where they wanted to go).
The inlet tube on the right varied by length among the ones I looked at. On my replacements these tubes were longer. I cut them to length and cut out the two square vents near the end. A pretty quick job with an angle grinder and a file.