The phrase "a problem that remained" is correct and usable in written English.
It can be used to describe an issue or difficulty that continues to exist despite efforts to resolve it.
Example: "Despite numerous attempts to address the issue, a problem that remained was the lack of communication among team members."
Alternatives: "an ongoing issue" or "a persistent problem."
In fact, the wall of the hoof became so thin that it was difficult to find room enough to anchor a nail for shoeing, a problem that remained with the horse for the rest of his days.
A problem that remained poorly understood until the 1960s is the relationship between the inverted images formed in individual ommatidia and the image formed across the eye as a whole.
A problem that remained troublesome concerns the fact that one often wants to assign probabilities to particular events, events that in the nature of things cannot be repeated in all their particularity.
A problem that remains uncertain or ambiguous will remain as such until it is defined, regardless of the approach that is used to address it.
This approach has been also applied to the solution of a particularly challenging problem that remained unresolved for over a decade [44].
For each encounter, we extracted the overview notes associated with a patient's problems that remained active at the time of discharge, suggesting that the patient should still take the medications for these problems.
Several problems that remained unsolved in the earlier discussion of Lewis structures can be unraveled.
However, there are still some problems that remained to be solved, such as high sampling rate and synchronous acquisition.
In both cases the control chart provided alarms for important problems in production and furthermore signalled problems that remained unnoticed by the layer managers.
However, despite the permissionless and self-governing nature of blockchains, the regulation and actual implementation of a decentralized system are problems that remain to be resolved.
In a recent post, I talked about some of the problems that remain in a specific (and familiar) part of the content-sharing sphere: file-sharing.
