Jabba the Hutt:
Your mind powers will not work on me, boy.
Why was Jabba immune to Luke's powers of the Force in Return of the Jedi?
It's entirely possible that to influence someone's mind, you have to have a pattern to do it with - you have to know, generally, how you want to manipulate their thoughts.
Jabba, as a Hutt, has a significantly different brain structure than humans (and human-like creatures). Luke is inexperienced at manipulating thoughts.
Essentially, to manipulate thoughts, a Jedi forms the thought that they want the other to have, and 'pushes' it into their mind. Experienced Jedi can do this easily, and can do it without obvious sign if they want. It's easier if the subject thinks along the same lines, or you have their attention. Qui-gon waves his fingers - drawing attention to himself - and speaks. Obi-wan, in the first movie, does the same, as a demonstration of the power of the Force to Luke.
Later, Obi-wan does the same to stormtroopers in the Death Star, without being obvious. He can do this because he's highly trained and experienced, and the thought is simple: "I heard something over there."
Against species with very different thought processes and/or brain structures (and chemistry?) the difficulty is increased significantly.
Bib Fortuna had a sycophantic personality - he was easy to manipulate because he liked to please others. Jabba was a crime lord, firm in his resolve and with a mind that was notoriously hard to change. He was in his seat of power, surrounded by things that reinforced his status and self-assuredness. He'd just demonstrated that beings lived or died by HIS will.
Then some meager little farm boy with delusions of granduer comes in, and tries to impose HIS will upon Jabba. Luke never stood a chance. Situation, physiology, and plot were all against him.
It's been stated in a number of sources, including 3 different RPG's, plus several other expanded universe sources, that some species in the SW universe are categorically immune to force effects upon the mind.
Even a weak willed Hutt is immune to the mental powers of the force. Jabba was a Hutt, this is why he's immune to Luke's efforts.
Certain species are naturally immune to jedi mind tricks. The species performing the trick can also be a factor. Since different species would have different types of brain wave patterns, it's logical to assume that certain species would be much harder to impossible for humans to effect. It's Possible that another Toydarian Jedi COULD have better success at influencing another Toydarian.
Or, it could be that said species are simply immune from such influences overall, or simply that Watto, Jabba and a few others were immune on an individual level and not because of some species wide protection, though Watto DID say "I'm a Toydarian, mind tricks don't work on me, only money". This seems to more strongly indicate that his particular species was immune rather than just Watto himself.