Definitely check the voltage. Some B&M's honestly don't know what they're talking about and don't care to explain what a battery or mod does when they're pushing it to you. Most are honest and will spend time with you, others just try to make a sell to push you out.
1). Check the voltage. The twist is adjustable, so he could have upped the voltage because he felt a 2.4 ohm coil would be better suited with a higher voltage (not always the case).
2). Make sure you give your wicks time to saturate. Top coils need more time to soak up the juice, so you can't always fill and vape, this is especially true on a new wick and coils. Top coils work against gravity as the liquid is moving up and if the juice has a higher VG content, it needs more time to soak as VG is much thicker than PG. Even 50/50 blends can take a little extra time.
3). Adjust your draw. If you're hitting it like a cigarette, ease back a little. If you're draw is hard, you could be pulling more juice off the wick than it can pull in, which leads to a dry and often nasty burnt taste. As a comparison, if you're draw is like trying to drink a thick milkshake, you're probably hitting it too hard. If it's like you're drinking a soda, you should be good to go.
As for recommendations, the Protank II is wonderful. It uses a smaller bottom coil, so you don't have the madusa looking wicks sticking out and it works with gravity, so liquid is pulled down into the wick as you vape. I own 6 of them right now and they are my primary tanks.
Kanger Protank 2 Pro Tank Glassomizer
Kanger ProTank II STNLS w/ Pyrex Fully Disassembleable - CLEAR [gv-ProTankII-CLEAR] - $21.95 : GotVapes.com, E-cigarette Supplies - Atomizers Cartomizers Mods Juice and more
Same price at both places, but they're both recommended.
All that being said, I've never had an issue with a top coil silicone cap being pushed down too much. Unless it's physically making contact with the coil, it shouldn't be an issue. If it is, you'll know it as it'll be melted into the coil. The stuff has high heat resistance, but not to the point where it'll withstand constant and continual heat from a mini heating element without some sort of damage.