I would try running the email through a testing service like Litmus. (I believe they have a free trial if you don't want to commit.) Those services allow you to visualize what your email looks like in different email clients and devices and could help you narrow down why the issue is occurring.
Without further testing...could it be that the email is set up to switch to a one-column layout on mobile, and that's why you're only sometimes seeing that layout when you test?
It may be set to switch to one-column but I am viewing it on the desktop. We have tried it in different clients and it shows up as one column.It does not do it every time either.