sticking point

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A point, issue, or situation that causes or is likely to cause an impasse.

from The Century Dictionary.

• noun Same as sticking-place.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A disputed issue or state of affairs that causes an interruption or outright impasse in progress towards some goal or resolution, especially in negotiation or argumentation.
• noun idiomatic, dated The point at which a process or thing, especially a state of mind or emotion, reaches its greatest strength and remains steadfast; sticking-place.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a point at which an impasse arises in progress toward an agreement or a goal