The weakness in proofreading ability can only be attributed to a shortage of staff.
Literal
This proofreading-capability [gen-の] weakness [topic-は] manpower [subj-が] insufficient [quotative-と] only can-think [polite-ませんね].
校閲力 'proofreading capability' uses the productive suffix 力 (りょく) 'power/ability' on 校閲 'proofreading.' ~としか考えられない 'can only think that' combines しか (only) with the potential passive of 考える — a strong-conclusion construction. 人手が足らない 'not enough hands' is the stock expression for being short-staffed.