It could be low refrigerant, but if that's the case, there has to be a leak someplace. There are other things besides low refrigerant that can cause poor A/C performance, however, such as a bad compressor. The only way to truly see what's going on is to do a performance test as stated in the service manual using a set of R134a A/C gauges and comparing the readings to specs for the given temperature and humidity. The best thing to do, if you don't have experience in air conditioning, is to take it to a professional. Your system had green dye installed in it from the factory. He could use a UV light and special glasses to try and locate the leak. One giveaway may be oil and dirt build up around where the lines connect to the compressor or condenser or around a nick in the A/C Condenser core.