troublesome problem

Father Paul had talked with him about it once more, and Franz had broached a very troublesome problem.

At that sight he felt what a philosopher feels at the sudden elucidation of a troublesome problem.

The most troublesome problem that the best breeders have is to get rid of the lower grades of their stock at a fair profit.

In many instances the accurate determination of the amount of graphite present in a rock has proved a rather troublesome problem.

We worked at the troublesome problem until three in the morning.