First off, the biggest thing I can recommend is to give tanking a try. I feel that everyone should level a tank, healer, and DPS class for the sake of knowing what those classes are and are not capable of.

To answer your question though, tanks DO have a number of enmity generating abilities, but when a mob runs off after a DD, especially a ranged DD, getting hate back can be a problem. They can't just stop what they're doing on all the other mobs to chase the monster around. However, the only ranged enmity generation a tank has both have their problems: Provoke only puts you slightly higher on the hate list than the DD, meaning it'll be back to chasing him if he does anything on it before you do, and it has a whopping 40 second recast. Shield Lob/Tomahawk cost around double the TP of any other attack, and require that the level 15 class quest be done. That's not a humongous requirement, but it does mean tanks who've just hit 15 and can join dungeons now may not realize they should pause to do their quest first, and quite a lot of legacy players may not be aware that there's a point to doing class quests besides xp and gil.

Due to the above issues, the most effective solution to getting hate back on the tank is actually something the DD can do: Drag the monster over to the tank. Then he can easily use flash, overpower, or tab over to it and use a combo to get hate back. If the DD is running around in the mistaken belief that it makes him dodge attacks more (It doesn't, it really really doesn't), the makes the monster quite difficult to hit with melee if chased down, and depending on positioning and number of monsters can make targeting it for a ranged attack more difficult.

In an ideal world, the tank wouldn't lose hate in the first place, but stuff happens. Sometimes the tank is new and needs encouragement, rather than being yelled at. Sometimes, it's a case of overzealous DDs who don't attack the tank's target, or who start AoEing everything right at the start of a pull. When this happens, it's the job of both the tank and the person who pulled hate to get it back where it belongs.

Of course, sometimes you get the tanks who don't know what they're doing and can't listen to advice. You can yell at them.