Usually, the landlord does not just answer as they should, but makes multiple, repeated objections to each and every question and request that you make, just to frustrate your efforts. This is a gamble by the landlord that you will not take the next step, but here you do, and the landlord and his lawyer face the possibility of money sanctions paid to you, as well as producing the information they tried hide, having the issued "deemed admitted" and otherwise suffering even greater expense and losses than they had ever dreamed would happen.

There is a two step process in most case, the first of which is to make a "meet and confer" effort to informally resolve the issues. This is legally required where they responded in some way, but not if they didn't respond at all.  The next part is to file the actual motion to compel or motion to deem admissions, as the case might be.