stay away from persei's and omicron's the carts tend to clog very often wasting oil and time. I own both and am not happy at all with them for this reason.
if you draw to hard on them, yes, you can draw the oil into the center tube and plug them up. once you get the little kink in NOT overdrawing on them (just sipping instead of taking a drag) your good to go. when not in use it can take a second to get the oil melted and rolling. just a small learning curve but they ll put you on your arse once setup. if its plugged plugged, it is likely the center tube on the cartridge is full of oil. once you get that cleared out, it ll work well.
Not saying others arnt good, but the Delta9 units are very well made, and work great, though not like most conventional e cig or oil vapes.
the Pic at the top of this page shows the Delta9 units, the Persei is the big fat one in the center, and has a single or dual cartridge top. it will also work on carts from 5ohm (weak) to 1.5ohm (hot). so you load it with two 1.5 ohm carts loaded with oil and plan on not moving for a while. lol
the Ophos is the curvy unit, and the Omicron V2 is on the far right. the one next to the Sharpie is the Original Omicron (V1) which was slightly changed and is now called the Ophos.