stumbling block
noun [ C ] uk/ˈstʌm.bəl.ɪŋ ˌblɒk/ us/ˈstʌm.bəl.ɪŋ ˌblɑːk/
障碍物；绊脚石
something that prevents action or agreement
Lack of willingness to compromise on both sides is the main/major stumbling block to reaching a settlement. 双方都不愿妥协是问题无法解决的主要症结。