What will cause an increase in the period of a simple pendulum that is swinging with small amplitude?

The time period of a simple pendulum which is oscillating is given by the following equation:

where, T is the time period of oscillations, L is its length and g is the acceleration due to gravity. 

Thus, the time period is directly proportional to the square root of the pendulum length. That is,

This means that in order to increase the time period of a simple pendulum, we have to increase its length. If the length of the pendulum is increased by a factor of 4, the time period increases by a factor of 2. 

That is, 

The change in the mass of a pendulum will not have any effect on the time period of the pendulum. 

Thus, pendulum length is the only variable on which the time period of a simple pendulum depends.

Hope this helps.