Just owning an investment property isn't good enough to deduct the expenses of that property (interest on your mortgage, utilities, and so forth). The property needs to be rented or available for rent.
For example, if a house or condo is purchased with the intention of being a rental but repairs need to be made prior to putting the property up for rent then the mortgage interest during this time period cannot be deducted. If two months goes by before the property is ready for rent and before the owner begins actively looking for tenants then that is two month's worth of mortgage payments that will not be deductible as interest expense on a rental activity.
A good second example would be a property that is available for rent but a tenant has not yet been found. If three months goes by before finding a tenant all is not lost because those three months' worth of expenses (mortgage, upkeep, utilities, etc.) ARE deductible. The key in this example is that the property was available for rent and the owner was looking for a tenant, even though he or she hadn't found one yet.
As a final example let's assume a house is purchased and the owners think that perhaps they'd like to make it a vacation home. If after a few months they decided vacations are few and far between and that they might as well lease the property out then they'll need to know that they can't deduct the expenses incurred on that property while it wasn't actively available for rent or rented out. It's more likely to be treated by the IRS as a second home or vacation home subject to different rules.
The lesson is to document that your rental property, if vacant, is available for rent and that you (or your manager) are actively looking for tenants!