You may be denied a visa for the following reasons:

When applying for a Tourist visa

You are missing a Tourist Voucher.
A hotel reservation is not enough for visa issue.

You put in your Visa application that you are going to visit friends or relatives. For a such visit you must have a Guest visa invitation.

You specify accommodation at a private apartment, Pay attention! A Tourist Visa means hotel accommodation

Dates of the requested visa do not coincide with the dates in the invitation

If one is currently unemployed and he can not prove the availability of funds for the trip.

When applying for a Business visa to Russia

The Host company that you specify in your Visa application is not the company that issued your Letter of invitation

You put in your Visa application that you are going to visit friends or relatives. For a such visit you must have a Guest visa invitation.

Dates of the requested visa do not coincide with the dates in the invitation

You state that you are unemployed.
Nota Bene! A Business visa is issued for a visit to Russia in behalf of your employer or business

You are applying for your visa too early (in the cases of visa directive via telex)

You specify the itinerary, unrelated to the formal host company.