There are several things that can cause you not to be able to get an acceptable SWR.
The first is the most obvious. Poor antenna, or bad coax. Make sure your coax is good. Make sure the antenna is mounted in a good location.
Some things I have seen that cause this problem.
- Wilson Little wil, almost impossible to get under a 1.6 SWR.
- Fiberglass antenna's, a break in the wire near the top of the antenna. Will show the antenna as being too short, and adjusting the top will do nothing. This antenna is no good.
- Common mode current. This is when the RF is traveling back down the outside of the coax, and or causes the coax to act like part of the antenna. SOLUTION. As close to the antenna as you can, take 6 to 8 inches of coax, and coil it into a tight coil, about the diameter of a spray paint can. In fact using a spray can to wrap the coax around works well. Use electrical tape to secure the coil so that it will stay tight and uniform. This will stop common mode.
- missing foil/vinyl membrane on bottom of magmount antenna. This is essential for the antenna to see ground plane.
- bad connections on coax connector.
- Cracked isolator on stud mount.
- incorrect procedure for taking SWR reading.
- Poor location of antenna. E.g. antenna's load not above roof of vehicle.
- Incorrect length whip being used.
- corrosion of any part of the antenna system.
There are some cheap antenna's out there that you just can't get lower then say a 1.5 or worse, like the wilson little wil. Wilson makes great antenna's. The 500, 1000 and 5000, as well as their silver load antenna's are excellent. I have been able to get them all to at least a 1.1 SWR, and some like the 5000, and silver load fiberglass whip to 1.0 using an analyzer. The Little will is just a poor antenna. Anything under 2.0 SWR is okay of a barefoot radio, although I don't like going over 1.5 my self. A barefoot radio, will do a 2 swr all day. If the radio is stock, and not peaked and tuned, 2 swr is fine and won't hurt the radio.
You should always try to get the lowest SWR that you can in any case. The lower the SWR, the more RF energy is leaving the antenna. As well, receive will improve the closer you get to a perfect match. This is known as being "On Que".
Check everything I have listed that applies. And if nothing fixes it, you may want to try a different antenna. Some antenna's just don't work well on certain vehicles.