What complications might arise with a deed of covenant?

[Azalea]

My family are trying to purchase a flat and we're almost at the point of exchanging contracts, but exchange has been delayed again because (from what I've heard) the signed deed of covenant returned by the landlord looked a bit ambiguous and our solicitor is not satisfied that exchange can take place yet. The deed of covenant is supposed to be the last hurdle for us before we can exchange, so what problems could there possibly be on the landlord's end? Surely all he has to do is sign the deed in the presence of a witness and have that witness fill in his/her details. Instead the landlord returned some "comments" and some other things - whatever that's supposed to mean! I assume that he either hasn't signed the deed properly or there's some vital information that's missing from him. Does anyone know what might have gone wrong? We've been waiting to exchange for weeks now and this is beyond frustrating.