I'm putting this in the panic forum to keep it with the other information threads.
All information travels down neural pathways to the brain and from the brain to muscles and organs. These pathways look like rope, but if you could look close they look like sausage links. Each link has a gap between it and the next. Situated around this gap are crossing points and the chemicals that bridge these points. The chemicals are called neurotransmitters and Serotonin is the most common. The largest number are in clusters in the brain. Each sausage feeds into one point on the next and that one might have a number of sausages feeding into it. It feeds into one point on the next. Hypothalamus directs these neurotransmitters to either carry a neuron across or not. Serotonin usually bridges the gap but can say no. Dopamine can override it because it can say yes or no. Dopamine is affected by mood. Serotonin does it's work through your whole body. Dopamine does it's work in your brain.
Although there are far more synapses in the brain it only has five percent of your Serotonin to work with. The rest is in your body and they are separated by the blood brain barrier that it can not cross. SSRIs don't build Serotonin, they keep you from wasting it. Too much and you get cognitive impairment, way too much and you get physical impairment too as it works on the synapses in your body.

Benzo's work by stopping information from crossing the gap and new ones are neurotransmitter specific. Some work best for things like restless leg because they can block instructions to your legs. Some opiates give you restless leg because they stimulate dopamine and dopamine can over ride Serotonin. Street drugs do the same thing. It isn't the street drug that makes you feel good but the increased dopamine production. Addiction happens because you want the feel good from dopamine that works with mood. Alcohol on the other hand is a depressant.

So why do some people need so much more than others and why do they not stop panic in some people?

Designer benzos work on specific triggers (like restless leg) But your doctor doesn't know which does what. So he/she guesses. Or uses Diazepam that works on everything. But Diazepam has a long half life so you need the smallest dose that works. Fine but there is still a reason why they don't sometimes work and that reason is you. Some pathways have to be open for you to function but anxiety can use these. If you use a benzo to get back to feeling positive that you were feeling before an attack they work, but if you are using them to block negative thoughts, you will have limited success. You will be overloading them. Distractions and coping skills work with them because the focus moves off the negative. So if you take a benzo to get the most out of it you want to think positive so it can do it's job without interference.

Ativan is a dangerous drug because it blocks all the crossing points but one making a priority pathway. Hypothalamus has to use that pathway only. When Ativan wears off all the other crossing points open without Hypothalamus's direction. Massive rebound as information floods across.
Diazepam blocks all crossing but not at all synapses so information is just slowed down as it finds detours. Don't worry, it would take very massive doses to shut down every synapse and kill you. Diazepam is usually used for withdrawal because it has a long half life and a blanket effect on the synapses allowing them to come back on line slowly and equally.