No matter how long I let it heat up never goes past 250 degrees

First, do the flames on the burners, when they're on high, appear to be normal? Or are they lower than normal, despite the burner knob(s) being turned to high? Do you turn the tank OFF after grilling each time?

If you answered yes, to the last two questions, here's what you need to do. Turn the tank valve OFF. Disconnect the hose from the tank. Wait 5 minutes and reconnect the hose. Turn the tank valve ON VERY, VERY SLOWLY, until it's completely open. Light each burner individually and turn the burner knob to high. Once all the burners are lit and turned to high, do the flames look taller than before?
If so, you just solved your problem. Remember to turn the tank valve on VERY, VERY SLOWLY each time you turn it ON.

If the above is not the problem, the grill thermostat may be faulty. To test this theory, purchase an inexpensive Oven Thermometer and place it on the grill grates, when preheating your grill. Close the lid of the grill and ten minutes later, take a look at the Oven Thermometer and compare it's temperature with the grill thermometer. If the grill thermometer is reading significantly lower than the Oven thermometer, you need to replace the grill thermometer. You can purchase one directly form Weber.

Hope this helped you solve your problem. Happy Grilling!