On a SoftGloss, the programs will vary somewhat, but all operate very similarly. We are looking at the bayswitch, backup air switch and the photoeyes mounted below the wheel scrubs as the vehicle leaves. When the vehicle exits, we need to see the bayswitch signal go "off". The backup air switch will pulse "on", then "off". After this happens, we should see the photoeye signal go "Off", then "On" when the tire blocks it. This happens for both the front and rear tire. If those signals happen in order, twice, vehicle detect is satisfied and you will see the "alternate arming device" output on the PLC go off for two seconds, then back on. If it does not happen in order, or is missing one of the steps, vehicle detect won't get satisfied until a timer in the program times out.
Most of the time when we hear of this issue, the backup air switch has failed. Look at the electrical schematic in the service manual that came with the machine to find that input on the PLC. Each time you step on the backup air switch, you need to see that input go "On", then "Off". If not, it is bad.
I can't tell you what inputs and outputs apply as they may be different depending on the vintage of your equipment.