I've heard of this model. No screen, just a dimple in the glass. My Vapolution uses a dimple too, but the bowl is so narrow a screen can't be used and it works very well as long as you don't try to use finely ground weed. I use pieces. Even with pieces too small to stay suspended in the bowl, all you have to do is turn it sideways. It seems dumb to me to go with this design on a standard sized whip though. Vaping temps aren't nearly high enough for a screen to be an issue.

You certainly don't want really fine ground herb with a dimple screen, and you shouldn't need it. It also shouldn't matter how much herb you have in the bowl. If you're hitting it hard that could be your problem. Pulling too much air, too fast through a vape will lower the temp to the point where it's too low to produce vapor. If you're getting either smoke or nothing, I can't think of any other explanation. If you turn on the heater in your car, and run the fan wide open, you'll lower the desired temp because the air is flowing through the heating coils too fast to get properly heated. Same thing with a vape.

As for how many hits (assuming you start getting any), it's pointless to try and count 'em. Too many variables-temp, weed moisture level and consistency, etc. And how big is a hit? A hit for me is 2 or 3 for my wife. When your bowl contents are brown instead of green and you're getting no more vapor it's done. And the consensus so far is that vapor doesn't get absorbed into your system as quickly as with smoke, and since vapor is so much smoother and cleaner, it makes sense to hold your hit longer than with smoke.

Try your vape with easy, slow hits. If it'll burn your herb, it'll vaporize it. You probably just need to get the right temp to the bowl, and the right temp setting is probably about half way between minimum and maximum.