phatalo is right. I think sometimes LMOs are rejected because the application is not properly filled out or the employer has not shown adquate evidence of recruitment efforts. Examples of recruitment efforts could include providing the number of times the job was re-posted, how many people applied(within the region/province & nationally), a checklist of what you(employer) was looking for vs what the applicants had/did not have, etc. It also helps if the employer is not involved in a labor dispute or laying off workers.
phathalo's point is that in the case of someone originally hired under PNP, the employer does NOT need to do any of that.
But on the LMO application form, where it asks about recruitment efforts, they do need to indicate that the employee was originally hired under the PGWPP and check "yes" on the box "intent to permanent"