The Data Views can at times have a delay in being populated. In my experience the Open view is the most likely to have delays. A recent release from Salesforce has added a new feature to reduce the amount of "false positives" in email opens and clicks that are the result of ISPs or MTAs opening and clicking emails (for SPAM and Phishing Detection, which has long plagued Marketing Cloud's reporting for large enterprise customers [in my experience]).
Double-check that the open tag in Marketing Cloud is correct. Previously I have seen people erroneously take their open tracking pixel from an email they received and insert it into their email template, which would record all opens against their own record. I mention this as your developers have recently re-coded the email. Re-insert it via the UI to be double-sure.
Be confident that the open tag is featured in the email and that some code isn't preventing it from being delivered. Best would be that you exist as a seed in your live send, otherwise send a test to yourself under the same conditions the live send was performed. View the source code of the email you have received (check your software manual) and find the open tag.
If the above doesn't reveal a problem after you've accounted for some possible delay, contact Salesforce Support.