When is it not possible to use algor mortis to determine time of death?

Algor mortis is the state in which a corpse's temperature is changing to match the ambient temperature of the environment in which it is located. It is generally not considered an accurate means to determine the time of death, because rate of temperature change is dependent on several environmental variables.
 
Below are a just few ways in which algor mortis could fail as a method to determine time of death.

The ambient temperature of the environment has changed (if it has gone from daytime to late evening, for example)
The corpse is wearing particularly insulating clothes, such as a down parka
The corpse is on a particularly heat-conducting surface
The corpse had an abnormal temperature at time of death due to fever or illness

 
For algor mortis to be an accurate means of determination of time of death, so many variables must be controlled. Often, when the need to determine time of death is present, it is not possible to control variables such as ambient temperature or what the person was wearing. Despite these shortcomings, algor mortis can be useful as a very broad estimation of a person's time of death. 
 
https://en.wikipedia.org/wiki/Algor_mortis

Algor mortis is the act of checking the temperature of a dead body and comparing that temperature with that of a healthy living person, since cellular activity stops at the time of death so no more body heat is produced to maintain the person's temperature.  The difference, given normal circumstances, can help approximate the time of death.  However, a number of conditions impact how reliable that is.  Anything that would influence a person's internal temperature or rate at which they lose body heat would impact the reliability of algor mortis.  Some common ones are:
How healthy the person was (did they have a fever that started their temperature at 103F instead of 98.6?)
How cold or hot the ambiant temperature is
What clothes they are wearing
The air flow in the room
If they were exposed to any sources of direct heat/cold sources
What's the body type
What were they lying on (impacts the heat transfer rate)
How was the body treated post-mortem (if the body was frozen and then thawed or placed in or by a heat source it would render algor mortis essentially useless)