WWII really didn't lend itself to the kind of widespread gas attacks that the Western Front is infamous for. It's not so much a question of resolve or efficiency but rather of logistics.

The main hurdle in deploying gas is simply delivering it. You don't want the gas blowing back on you and so you need to contrive some way to deliver it as close to the enemy lines as possible (you can't just have your men run up and lob it at the enemy like so many hand grenades). This means you need to either deliver it by artillery or aerial bombardment. If you're delivering it by artillery, you're probably working with 150mm howitzers and, if that's the case, you can only deliver about 3kg of the stuff per shell (based on German gas canister sizes) -- for comparative purposes, total shell weight for conventional high explosive (HE) shells in that class ranged from about 40kg to 55kg.

If a given shell contains about 3kg of a gas, and you need several hundred tons to effectively inundate an enemy position of any appreciable size (because gas isn't really a localized weapon), you're looking at several thousand artillery shells being fired. Since you need the gas to all be delivered to the target in a relatively short timeframe (so it doesn't just blow away before reaching the concentration necessary to inflict serious injury), and considering howitzers in that class could only average about one shell every 20 seconds, you're going to want hundreds of field pieces firing the gas canisters at once.

If we're talking about the Western Front, this kind of artillery concentration wasn't terribly difficult to find. At Verdun, over a period of about nine days during a particularly bloody phase of the battle, the French fired approximately three million artillery shells. The Western Front featured some truly mind-boggingly huge artillery bombardments. In the face of that kind of firepower, earmarking a few hundred pieces for dropping gas is a relatively small investment.

But in WWII? That's no longer really the case. While WWII featured significantly more artillery being deployed, the times when armies would essentially line up and try to bludgeon one another to death were essentially over. That certainly happened at times (Stalingrad), WWII typically didn't have the kind of static positions that lent themselves to massive artillery barrages. Those static positions that did exist usually involved some form of coastal defense (the Atlantic Wall and Japan's network of fortified islands being the most obvious examples), neither of which were particularly easy targets for huge quantities of artillery.

There was also the risk of gas backfiring on the user. The cases where wind shifted in WWI and gas blew back on the party that launched the attack are pretty well-known; gas is a fairly fickle weapon. There was also an incident where the Germans unintentionally blew up an American freighter carrying mustard gas off the Italian coast. While a bit of an unusual example, this shows how gas can also be a liability.

On top of all of these problems inherent to using it, there was something of an informal agreement: "if you don't gas me, I won't gas you." As long as both sides had reasonably equal access to chemical weapons, neither side particularly wanted to deal with being on the receiving end and thus abstained from using the weapons (because using them would provoke a retaliatory strike along the same lines). A similar line of thinking would eventually give rise to the MAD doctrine. When there wasn't a risk of retaliation, countries generally didn't show much restraint. Japan used gas against China largely because the Japanese didn't have to fear retaliation (and also because the war in China was often a much less fluid affair than the war in North Africa and Europe).

All told, it was more difficult to organize gas attacks in WWII than WWI, using gas would almost certainly result in a retaliatory strike (which the major belligerents learned in WWI), and the efficiency of a gas attack was often reliant upon the enemy not being prepared for it (and also upon weather, which isn't exactly the most predictable thing in the world).