A race arena are a small area. People usually have to use a very small antenna to not get their scanners overloaded and then loose sensitivity. You tried using the ATT feature to check that you didn't get de-sensed? The users of the 400MHz frequencies must have huge problems if the signal levels where that bad. Most likely your scanner was swamped from all the RF transmitters in the area.
The only way to make an antenna with higher gain are to do it more directive either by only having it receive from one direction or to reduce the signal it receives from the sky and concentrate it more to the horizon. That means that you have to keep the antenna vertically. A portable scanner are often held in you hand with the antenna pointing halfway down to a horizontal position and will then function worse than a non gain antenna that receives well from almost any positions its held.
Upmans test of antennas indicate that all antennas he tested worked better in the 400Mhz range than the stock antenna.
http://massivelyuninformed.me/measuring-relative-antenna-receive-performance/
/Ubbe
