Make sure that the batteries contain enough power. If not change the batteries. 
Clean the shaver according to the instructions in the user manual.
Especially check if any long hairs are stuck under the trimmer.
Tip: If you use the appliance frequently, replace the shaving foils every year and lubricate them twice a year with a drop of sewing machine oil.
If the appliance still does not work very well, contact your dealer or the nearest Philips service center or visit www.philips.com/support.