stumbling block

describes a very important or serious obstacle

describes the primary or most important obstacle

describes a large and significant obstacle

describes a central or crucial obstacle

emphasizes that something is a genuine and serious obstacle

describes something that could become an obstacle in the future

indicates what is being obstructed

to encounter an unexpected obstacle

to function as an obstacle

to be shown to be an obstacle over time

to turn out to be an obstacle

to develop into an obstacle

to eliminate an obstacle