The phrase "a more difficult problem" is correct and usable in written English.
It can be used when comparing the difficulty of one problem to another, indicating that the problem in question is harder than a previously mentioned one.
Example: "After analyzing the initial issue, we realized that we were facing a more difficult problem than we had anticipated."
Alternatives: "a tougher challenge" or "a greater difficulty".
What to do about SCAF has been a more difficult problem.
The apartment houses on the border presented a more difficult problem.
A more difficult problem was restraining Pope John Paul II, a Pole who loathed the Russians.
Sudden death usually occurs outside the hospital, of course, and thus presents a more difficult problem.
A more difficult problem arises in combining the price relatives for different commodities.
The forest reservoirs of the virus present a more difficult problem, but the combined use of immunization and insecticides did much to bring this disease under control.
Changing the structure of an economy to make it more competitive is a far more difficult problem than establishing a permanent bailout fund, or even forcing austerity on countries with large deficits.
Instead, the algorithm increases the amount of resources available in order to handle resource requests while also considering stochastic failures of system resources, thus solving a much more difficult problem and leading to a more robust system design.
As a result, multicast design in DTNs is a considerably more difficult problem compared to that in Internet and mobile ad hoc networks.
Today's strategic planners confront a much more difficult problem: how to succeed in an environment of greatly reduced public fund-ing, through smaller, incremental implementation approaches which, though more limited in scale, produce tangible benefits for urban communities.
The point is (and what Gerber and Green and a legion political data wonks have taught us) getting voters to the polls is a much more difficult problem to solve than just slipping a mailer under their door on November 4th.
