The USPTO has started to notify applicants that their national stage applications are retroactively abandoned if an oath or declaration were missing at the time of filing of a request for continued examination (RCE).
37 C.F.R. 1.114, which set forth rules relating to RCEs, states that its rules do not apply to an "international application that does not comply with 35 U.S.C. 371.” Because 35 U.S.C. 371(c)(3) sets forth that an applicant for an international application shall file "an oath or declaration of the inventor," the USPTO has begun to require that an oath or declaration need to be filed in a national stage application before an RCE can be submitted. This is a departure from USPTO's prior practice of allowing an oath or declaration to be filed up to the date of payment of the issue fee.
Make sure to check that an oath or declaration in the national stage application has been filed before submitting an RCE. Otherwise, you may find yourself having to submit a petition to overcome the abandonment of the case.