You don't say whether it's an old machine, or any of its service history, but here are some options you can try yourself:
1. The water inlet float valve isn't seating properly, and warm, outside water is dribbling in constantly. This will make it hard for the machine to lower the water temperature enough to make ice. Check the float valve to make sure the flow of water completely shuts off when the reservoir is full. If it doesn't, either clean off the mineral deposits that are stopping it closing, or replace the valve unit.
2. The condenser coil (it's a bit like a car radiator, inside the machine) is so dirty that air won't flow through it efficiently, causing the temperature and pressure to rise in the system. The machine needs to lose heat in order to make ice. If the condenser coil is dirty, the heat stays inside the system and you won't get ice. Clean the coil thoroughly. If it's filled with dust and lint, use a vacuum cleaner; if it's full of greasy dirt, use a de-greasing liquid.
3. If the machine is trapped inside a cupboard or other confined space, it won't be able to lose heat efficiently, and for the same reason as (2) above, you'll get no ice.
4. If it's none of the above, and you're not a refrigeration technician - call one. (You won't be able to do it yourself). He will look for:
a) Low refrigerant. And he'll want to know why - no point in just filling it up until he knows where the leak is.
b) Compressor's not compressing. If it's a new unit, he'll want to find out what happened to break it, before installing a new one.
c) Compressor's short cycling on and off. Check the capacitor, the relay and the bi-metal switch.
d) Control board error. Change it, and its sensors.
Good luck.