The seals are a wear item of sorts....I had one go bad on my 6 month old trials bike for no apparent reason.
You might also just have a piece of grit in the seal, you can cut a thin credit card or piece of plastic into a small flat hook, and pull it around the seal and see if it will remove the grit....sometimes it works just fine.
http://sealmate.net/Taller shocks, your weight and the windshield may put a bit more load on the front end and make the fork springs a bit less able to hold the front end up - but it would not make too much difference in the operation of the seal.
To test the "sag" as Gary_in_NJ has recommended - but a small nylon tie around a fork tube and slide it down until it touches the seal, then sit on the bike being careful not to pull on the front brake or move around in a way that would compress the forks more than just sitting on the bike does...you may have to have someone help you hold the bike upright as both feet should be on the pegs. Then carefully get off the bike so as not to compress the forks, and pull the forks up all the way...and measure the distance between the nylon tie and the fork seal. That distance is the sag...and it should be near the distance that Gary suggested. If the sag is bigger....you need to make longer spacers for the fork springs or add some washers to put more pre-load on the springs.