The phrase "a difficult problem when" is correct and usable in written English.
It can be used to introduce a situation or context in which a problem becomes challenging or complex.
Example: "It becomes a difficult problem when the resources are limited and time is of the essence."
Alternatives: "a challenging issue when" or "a tough situation when".
"It is a difficult problem when you have a lack of oil," he said.
But it's a difficult problem — when we put someone in charge of enforcing the law, there is a measure of trust there.
This format really creates a difficult problem when you are dealing with someone like Ben Revere, who just got sent down this week.
Modelling transport and long-term creep in concrete materials is a difficult problem when the complexity of the microstructure is taken into account, because it is hard to predict instantaneous elastic responses.
However, action recognition still remains a difficult problem when dealing with unconstrained videos such as web videos, movie and TV shows, and surveillance videos.
Nevertheless, texture analysis remains a difficult problem when applied to color [6], multi or hyperspectral images; where image pixel takes its values in a multidimensional space.
This may pose a difficult problem either when agents receiving those goods have private information in regards to their values or needs.
Monitoring pulse oxygen saturation (SpO2) and heart rate (HR) using photoplethysmography (PPG) signal contaminated by a motion artifact (MA) remains a difficult problem, especially when the oximeter is not equipped with a 3-axis accelerometer for adaptive noise cancellation.
Due to the degeneracy of the nucleotide content among motif instances, pattern matching-based in silico motif search in higher eukaryotes remains a difficult problem, even when using formalisms such as the position weight matrix (PWM) (or nucleotide distributions at each position of the motif).
One of the most difficult problems when using eye tracking to identify the focus of visual attention is the inaccuracy of the eye trackers used to measure the point of gaze.
This Congress has shown they can find solutions to difficult problems when the pressure is on.
