The participial adjective concerted is traditionally defined as planned or accomplished together. It comes from the noun concert, where concert refers to an agreement between two or more people, so something done concertedly involves the efforts of multiple people. The word is synonymous with coordinated, united, combined, collaborative, and joint. So, given the traditional sense of concerted, the common phrase concerted effort is illogical when referring to the effort of a single person, and it bears replacement in this and similar phrases with alternatives like determined, resolute, dogged, and steadfast.
Still, people seem to love using the phrase concerted effort, so there is probably no point in trying to promote logical alternatives in this case.
In each of these sentences, concerted effort is questionable because it applies to a single person or entity:
Rafael Cabrera, head of the operators’ union, said he hopes the next Muni chief will make a more concerted effort to communicate with frontline employees. [San Francisco Chronicle]
Opec members failed to agree an official increase in production quotas despite a concerted effort by Saudi Arabia … to boost output. [Financial Times]
But lately he’s been making a concerted effort to burnish his social conservative credentials. [NPR]
And concerted is more logical in the following instances because it describes joint efforts:
After a concerted effort by his fellow Democrats to push him out, Weiner finally decides to resign from Congress. [Salon]
So we should probably make a concerted effort to help them arrive here in a nearly sane state. [Montreal Gazette]
Differing opinions can be dismissed as trolls and there is a concerted effort to bring people into line. [New Zealand Herald]