1
general::
Phrase(s): the weak link (in the chain)
Fig. the weak point or person in a system or organization. • Joan’s hasty generalizations about the economy were definitely the weak link in her argument.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
The least dependable member of a group, as in The shipping department, slow in getting out orders, is our weak link in customer service, or They're all very capable designers except for Ron, who is clearly the weak link. This expression alludes to the fragile portion of a chain, where it is most likely to break. [Mid-1800s]
American Heritage Idioms