"difficulty may occur" is a correct and usable sentence in written English.
You can use it when you want to express the possibility that a problem or difficulty is ahead. For example, "We must consider that difficulty may occur in the execution of this plan."
More difficulties may occur as early as next week, when the House could take up the president's energy program, including a proposal to drill for oil in the Arctic National Wildlife Refuge.
Difficulties may occur by inhomogeneous heating of the brake discs.
However, if the scalar potential formulation is utilised, convergence difficulties may occur.
Similar difficulties may occur in areas where there are stable swidden farming systems in place.
In some cases when the algorithm is applied to problems with discontinuous probability density functions difficulties may occur with convergence.
Further difficulties may occur in the case of partnerships between different Local Action Groups, such as disagreement about whether a given factor is a Strength or a Weakness, lack of information about the relationships between Strength and a Weakness factors and decision alternatives, as well as impossibility of ranking the decision alternatives.
In smaller litters, difficulties may occur during labour due to over-sized pups.
Estimation algorithms for the mean ODE approach are straightforward to implement although for higher dimensional or less stable systems more difficulties may occur.
Although the most appropriate model for studies of periodontal disease pathogenesis in experimental primates appears to involve the application of silk ligatures, some difficulties may occur in establishing periodontal tissue breakdown by using this model.
If the excitation spreads to other areas, other neurological symptoms — like numbness, tingling or difficulty speaking — may occur.
