Imap not returning emails while scaning inbox

Hi,
  am using Imap I have an emails in my account
I have scanned emails around 11 am IST but am not scanned emails.If i
put scan_since with previous day like 24.hours.ago then am not get
results

I have started scan emails again in the afternoon around 3pmISt.Now am
able to scan emails with 6 hours.ago .Now emails scanning only after 12
Pm.Not scanning emails which i received around 11:30Am

Can any one face same issue.Please share your experiences.
My intention is should scan all emails which i received today and should
work scan in hours i.e like 6 hours.ago,8hours.ago etc..