Sometimes there's nothing wrong. The number on the sidewall of the tire is not the amount of pressure the tire is supposed to have. Some models require a different amount of pressure than specified on the tire. Check your owner's manual. Some manufactures will have that listed on a sticker under the seat.
Pressures are different for the front and rear of the motorcycle.
The front is always less than the rear. That means before the tires get heated up from riding. It won't hurt to check them once a week. Purchase a good quality gauge. It will last for years, and the extra mileage and improved handling will be well worth the time you spend checking the pressure.