Assuming the landlord is a limited company...
There could be a number of reasons such as a director having resigned or being struck off or some other action prevents him from signing documents eg an injunction. Maybe for some reason there are only certain authorised signatories and he isn't one of them.
A property transfer must be by way of a deed and signed as a deed, in the case of a company by either 2 directors or 1 director and a company secretary (you can't have a limited company with one sole officer (ie director or secretary), there can be only one director but must be a company secretary or other director). Unless validly executed the transfer is not valid.
Another reason could be in the company documents. Each company has an articles and memorandum of association which set out what the company can and cannot do.
Could be the company is not allowed (ie there is not power in the documents) for them to transfer property or similar. It is usually a standard clause but could have been taken out for some reason.
There are also some company formalities which may not have been carried out as yet such as if any resolutions, meetings etc... are required.
As you are, I assume, selling a lease of your flat, if you are also now getting the freehold I assume this is what will now be transferred to your buyer as in law you can't be your own landlord (which you would if you owned the lease and freehold) so the two merge in common law so the lease effectively disappears and the freehold remains which would then be transferred to the buyer.
If this is the case then the transfer from the current landlord of the freehold interest would need to take place prior to exchange as the contract will for the above reason, I'm assuming, be for the freehold title, so you must own it before you can become legally bound to selling it.
Probably best to