Criminal case files are not reviewed before the first court hearing in too many instances, the prosecutions watchdog said.
The finding emerged in an inspection examining the CPS's work in relation to efforts to improve the magistrates' courts system.
Overall, it concluded that the service is making a positive contribution to the transforming summary justice initiative.
The programme was adopted by criminal justice agencies to reduce delays and hold fewer hearings per case in magistrates' courts and includes a requirement that all cases are reviewed by the CPS before the first hearing.
Inspectors observed 19 magistrates' courts sittings, and assessed 271 files.
HM Crown Prosecution Service Inspectorate said that CPS charging decisions were found to be good but that there "was a failure to review cases for the first hearing in too many instances".
In 37.7% of the cases it examined there was no evidence that a review had been completed before the first hearing.
It added: "Both the quality and timeliness of initial review by the CPS needs to be improved."
The watchdog also said the CPS is failing to "engage effectively" with the defence prior to the first hearing.
A CPS spokeswoman said the report acknowledges it is "making a positive early contribution to the transformation of magistrates' court work".
She added: "We are putting in place measures to ensure we always record the review of our files.
"We have introduced an app for prosecutors which assists and prompts them to record a review electronically.
"Our compliance and assurance team is also carrying out sample checks of cases, and will inform areas if review records are not being made, so that they can address issues at an early stage."
The report assessed 81% of first hearings as effective, adding that generally the correct personnel were present and CPS prosecutors were well prepared.
Chief Inspector Kevin McGinty said: "Our inspection found that the CPS engagement with this initiative was positive, but it is essential that the focus is maintained and momentum is not lost."
He said it was too early to say if the initiative is leading to the level of improvement hoped for, adding: "It will be several months before this can be realistically and accurately judged."
Chief Crown Prosecutor Barry Hughes said: "A lot of work has gone into ensuring that our prosecutors are provided with the necessary training and support, and we are pleased this has been acknowledged within the report.
"Whilst the report is encouraging, we are committed to further improving how the system works, alongside the police and court service.
"We are already taking action on many of the report's recommendations and will continue to work to ensure victims and witnesses do not face unnecessary delays and hearings, and to increase overall efficiency in the magistrates' court."