there(in) lies the rub
colloquial phrase

- something that causes a difficulty or problem

Merriam-Webster

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ORIGIN

This phrase was famously used in Shakespeare’s Hamlet and is now seen in many different variations, including: there(in) lies the rub, there’s the rub, that’s the rub…

“To die, to sleep;
To sleep: perchance to dream
ay, there’s the rub
For in that sleep of death what dreams may come,
When we have shuffled off this mortal coil,
Must give us pause: there’s the respect
That makes calamity of so long life.”

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SYNONYMS

THERE’S THE RUB, most important, main issue/question/difficulty, crux of the matter, key concern, essence, quintessence, bones/heart of the matter, heart and soul, centre-piece, meat and potatoes, long and the short, essential/focal point, cornerstone, sum and substance, sine qua non, numero uno, motivating force, salient point, name of the game, nature of the beast, be-all and end-all

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PRACTICE OWAD in an English conversation, say something like:

“Jim will be working for our competitor - but HERE’S THE RUB - he hasn't signed an NDA.”

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Paul Smith