Odd Starting

1998 Chevrolet S-10 V-6 4x4
When my truck sits for more than approx. 20 mins. It is hard to start(turns over a while). Once cranked, it runs fine. I've checked the fuel lines and replaced the fuel filter. The battery/alt. Are strong and both are only a yr. Old. I have also gone through routine maintenance, since it was time (oil change, air filter, PCV valve).