The phrase "a rather difficult problem" is correct and usable in written English.
It can be used when describing a problem that is challenging or complex in nature.
Example: "The team faced a rather difficult problem when trying to integrate the new software with the existing system."
Alternatives: "a quite challenging issue" or "a somewhat tough dilemma".
Finding exact geometric properties characterizing sets with optimal admissible meshes appears to be a rather difficult problem.
Detecting the presence of resonant SMS waves in a plasma of high dissipation level is a rather difficult problem.
Faced with so many discrete choice models, how to choose an appropriate one to simulate travel behavior is still a rather difficult problem.
Thus Russell has set himself a rather difficult problem, since it is not at all clear that there can be any true propositions that are not, in some sense, propositions about what is, has been or will be.
Because the ray direction does not coincide with the normal to wavefront and the index of crystal changes with the direction of propagation, the design of crystal components is a rather difficult problem.
Multiphase flow and displacement can pose a rather difficult problem, both because the underlying physics is complex, and also because standard laboratory investigation reveals little about the mechanisms that control micro-scale processes.
One group adopted a practical approach to this rather difficult problem.
However, unless the cover model is a complete statistical descriptor of the empirical source, such optimized schemes may, paradoxically, end up being more detectable if the warden designs the detector 'outside of the model'[6, 7], which brings us back to the main and rather difficult problem - modeling the source.
It is a rather difficult philosophical problem to discuss the (possible, non-technical) nature of such "truth degrees" or "truth values".
Cyclic operation, although not planned, has demonstrated its viability as a practical, although not the optimum, solution to a rather difficult, unforeseen problem.
Daniel Berleant agrees, stating the current difficulties of "mobility is undeniably a rather difficult technical problem, and computers are more likely to manipulate data better than humans than they are to take over most manual labor jobs, at least for the time being".
