You are going to have to check the mail headers if you want to know the answer to this.
The second post in the page below shows how to view the headers on gmail
http://www.ummah.com/...-Gmail-and-Hotmail%29
Then you need to follow the received by sections, starting from the earliest working down to the latest and try and spot at which point there is a big gap in time, and that is where your mail is getting delayed.
If you are having trouble understanding the header then you can also copy and paste the mail header into this page, click submit and it will tell you all the servers the mail passed through and what the delay time is.
http://mailparse.co.uk/