I'll try to break it down as best as I can without getting into the physics of it all (partly because I don't get that part of it myself!):
LR = Low Resistance - the coil has a lower resistance, meaning it's capable of producing more heat on a 3.7v battery. If you use a battery that is higher voltage than that, the coil cannot take much, and will probably pop. The benefit? It produces warmer and fuller vapor since the coil is hotter, which simulates a high-voltage experience, but doesn't burn through as much
juice
SR = Standard resistance - Middle of the road in terms of what the coil creates - Best suited for 3.7v to 6v, although sometimes 6v is too much for certain juices to handle at the moderate amount of heat the SR atty throws off
HR = High Resistance (some times these atomizers are referred to as high voltage) - Doesn't produce as much heat as SR - generally you need at least 5v to get it to work. 3.7v won't even heat the thing - This is for use really at 6v or 7.4v - creates VERY warm vapor, but will typically leave you without much flavor as it still will burn very hot at the coil
Now, to get back to the OP's question, it may need to be broken in. Also, if you've been using atomizers that are SR, they typically run at 2.5ohm to 3ohm resistance. An LR may be 2ohm, which isn't low enough to really get a juice
vaping like a higher voltage device. Typically, LR's are 1.5 to 2.0 ohm. I like 1.5ohm the best fo rmy LR's. Your atomizer may need some break-in, especially if it was very primer coated when you received it.
The only real way to know how much lower the resistance is requires the use of a multimeter.