The phrase 'major source of difficulty' is correct and usable in written English.
You can use it when you want to emphasize a challenge or problem. Example sentence: Finding a parking spot in the city is a major source of difficulty.
However, some of the features that make supervisory control an attractive paradigm to solve theoretical problems are often a major source of difficulty in implementing a controller: such is the case, for instance, of the abstraction level usually considered in SCT.
Another major source of difficulty is missing (latent) variables.
Word misspellings are a major source of difficulty in de-identifying free text.
As in our investigation, the issue of double-barreled items was a major source of difficulty found in the IKDC.
Loneliness is a human condition which is widely recognised as a major source of difficulty for older people (Cattan et al. 2005, 2009; Victor et al. 2000).
The non-convexity and non-monotonicity of F are major sources of difficulty for nonlinear solvers of coupled multiphase flow and transport in natural porous media.
Fluctuating cognition, hallucinations, and delusions are major sources of difficulties and distress for both patients and caregivers.
In the end, the committee failed, and many Democrats pointed at Mr. Kyl as a major source of the difficulty.
However, the underlying distributions for all three question sources on the measure's relevance, fluency, and ambiguity show that all were similar and all were probably not a major source of pedagogical difficulty.
The major source of uncertainty concerning their use is the difficulty of extrapolating study results to different contexts and under different conditions in the future.
Eating and drinking difficulties are a major source of ill health and stress across the stages of dementia in multiple settings.
