Apart from what the others have said about the enlarger filtration, lightstruck paper etc, other issues might affect contrast:
Freshness of developer - should be made up fresh for each session. Developer, especially diluted, can quickly oxidise (exhaust) if not kept in an airtight container. Make sure you tighten the top on your stock bottle properly - better still transfer it to a reliably airtight bottle; fizzy drinks bottles work very well - with usual precautions if you have children/vulnerable adults around. If stock dev looks dark brown it's probably oxidised and won't work properly.
Temperature of developer; developer should be at around 20c, if it's cooler you should develop for longer than 2 minutes (and always develop to completion; don't snatch the print early!) - it should still work though.
Contaminated developer; developer contaminated with stop bath or fixer will show signs of exhaustion (see above).
Last edited by kevs; 10-13-2012 at 09:37 PM. Click to view previous post history.