First thing you need to do is verify it's positioned properly on the trailer. To do this, have it loaded just like you have it when going fishing, gas in the tank and all. You want your normal load, right where it goes, and the motor tilted up and set in place for towing. Now take the boat to a feed supply or truck stop that has scales and weigh it with it just like you would have it fishing.
Next you need to make sure you have your tongue height set properly on the truck. Check with you trailer manufacture but 13.5" is a good height on most. You set that by using different size drops on your receiver.
Now you measure your tongue weight, for smaller boats this is no problem, just set a set of bathroom scales on something at the right height and set the tongue on them. Heavier boats can get fun trying to find a way to weigh them. Two set of scales side by side with a 2x4 across them and set the tongue in the center of the 2x4 will work most times. For conventional trailers with ball-mounted hitches, proper tongue weight is roughly 10 to 15 percent of the total loaded trailer weight. For instance, if a 2,000-pound conventional trailer is loaded with 1,000 pounds of cargo, the proper tongue weight of the loaded trailer should be between 300 and 450 pounds, or 10-15 percent of the loaded 3,000 pound total.
Anyway, once you get a way of checking you tongue weight, you position the boat on the trailer by moving it forward or backwards until you have the proper tongue weight. Once you have that, you position your front stop so that it locks the boat in place at that point. Then you adjust your bunks and center rollers to ensure they are positioned to properly support the boat.
Yep, it's a little bit of work, but I highly recommend you do it. I learned the hard way that dealers don't do a very good job at setting up trailers. The very first new boat I bought had the hull ruined by the way the dealer set the roller on the trailer. One roller was too high and after about eight months, put a hook in the hull. I noticed my boat was slowly loosing speed and after about eight months it had lost almost five mph. I was changing props and everything else trying to figure out why, until I found that that hook.