Why do polar bears struggle getting food?

There is no harm or decline in the population of seals. Its just that seals usually spend most of their time under the ice sheet in the below freezing temperature of water below.

Polar bears have to wait for a long time near the breathing holes that seals make before they get under the ice.When the seals come out to take in air again, the polar bears kill them.

Since, they can stay underwater for hours together, it may seem that they are few in number.