+1 what GarBow has said plus the following; -
If an alleged non-compliance with Planning Permission or breach of planing law a is "under consideration" then the Local Authority may not want to give out any information until the matter is resolved.
They may cite that as the reasonbut I think you may still be able to view some of the documents on the file, but perhaps not online.
The issue is more properly referred to as Planning Enforcement action, as opposed to a compliance issue - the latter gives rise to the former after formal referral.
Usually this starts off by someone writing to the local authority bringing the alleged lack of compliance or breach of planning law to their attention.
The local authority usually issues a warning letter, which tells the owner of the premises that enforcement action is being considered.
This may bring the matter to a successful conclusion by itself.
Failing this, the local authority usually goes ahead and investigates.
If nothing is found, the LA confirms in writing to the owner and referrer that the develoment is in compliance or does not constitute a breach of planning law, and that no further action is being taken.
If an issue of non-compliance arises, the local authority will make an internal decision to proceed or not, because sometimes these things end up in court.
It then issues the Enforcement Letter, stating the non-compliance/breach is and requiring it to be remedied.
Depending on the speed of response of the perpetrator and the diligence of the local authority, things can drag on for months or be resolved quickly.
The end of the affair may result in an application for retention permission [which can kick off a whole planning process] or the matter going to Court [which can kick off a whole legal process].
HTH
ONQ.
[broken link removed]