Typical range is 65-75% of the maximum range due to terrain, weather, buildings, cables, hills/valleys, power lines, etc. If you are in a high-influence area, for example near big radio-emitters or high-power-lines, this can reduce greatly the range. The same can happen if you are on the top of a hill while your dog is in the valley, or vice versa.
If you are experiencing less than 50% range or if these environmental factors have been eliminated:
Try your trainer in a different area.
Replace the batteries, or if they are rechargeable, ensure the collar and transmitter are charged.
Ensure the remote transmitter and the collar are correctly synchronised.
If your remote has an antenna that can be screwed on, make sure it is tight.