Mailerlite integration - missing contacts

[DT2]

I have set up an integration between Hubspot and Mailerlite. It is generally working fine but I've noticed a handful of contacts are not syncing from Hubspot to Mailerlite. There is no reason I can see why they wouldn't. There is no difference between them compared to others. They have email addresses, names included and are on the correct Hubspot list that I have set it up to sync, they just don't appear at all in Mailerlite whilst others in the same list do. 

Any ideas of what I should look at/try to work out why these individual contacts are not being transferred?