WordNet sense 1 (not responding to treatment):

refractory

WordNet sense 2 (not tractable; difficult to manage or mold):

intractable

WordNet sense 3 (devoting full strength and concentrated attention to):

determined

WordNet sense 4 (stubbornly unyielding):

pertinacious, tenacious, unyielding

WordNet sense 5 (stubbornly obstructive and unwilling to cooperate):

cantankerous

WordNet sense 6 (tenaciously unwilling or marked by tenacious unwillingness to yield):

obstinate, unregenerate