Any service standards for allegedly snot straightforward applications

[Crowbing]

I submitted two applications for my two kids under 16, FROM OVERSEAE, to extend their Visas that expired on 15th Jul 22. Both applied on the same day, and biometrics taken on the same day. Supporting evidences are almost identical, being minor siblings of same parents. My daughter's application was concluded and passport ready to collection after 9 days. However,, on Day 15, which is last day of service standard published they sent an email saying that my son's application could not be concluded within the 15 working days because it was not Straightforward!!! I believe something wrong going on or simply it is covering up for failure to finish workload within time. Now it has been 30 days since they sent that email. they never contacted me for further explanation or requests. I did send several complaints to the relevant emails; Pubic.enquiries, complaints and ukvinhsteam . All @ homeoffice.gov.uk but never had any reply.
I need to know:
if there are any time limits to process these applications being not straight forward
and
Is there any legal action that can be taken against UKVI?
regards