There's no reason to believe your counter isn't working properly, but I like to do the simple test shown below every now and then just to make sure. All it checks is that two fairly long rides of N wheel revolutions will result in exactly the same number of counts. This should always be the case regardless of road surface or tire pressure.
1) Roll your bike forward until the valve stem of the front wheel is next to the ground. Record the counts on your counter. Call it count_A.
2) Ride your bike until the counter reads (count_A + 20000).
3) Roll the bike forward until the valve stem is next to the ground. Record the counts shown as count_B.
4) Ride your bike again until the counts shown is [count_B + (count_B - count_A)]. The valve stem should again be next to the ground.