There are 2 issues which are likely to affect you here:
The Performance of AF-C
AF-C is not magic, and it will not follow the movement through the frame. Your focus point needs to remain in the same place in the frame throughout the shots. If you're off by even a small amount, it could lead to a big difference in where the focus lands. Check your images - can you find the point the image focussed on?
AF-C simply may not be fast enough. The Nikon D3200 is an entry level camera - the speed of AF in general, and the AF-C tracking will be considerably lower than in the pro bodies which are designed for sports. Motorsport in particular is very challenging because of the speed of movement. If you're close to the track the 'closing speed' between you and the cars will be very fast, and probably faster than your AF can cope with.
f/1.8 gives a really shallow depth of field
With such a shallow DOF, getting accurate focus on stationary subjects can sometimes be a real challenge - with a moving target, you'll really struggle - ask yourself if you actually want a DOF that small?
Getting Better Images
With fast moving subjects like this - the best way of getting a good focussed image is to pre-focus on a particular spot. If your cameras 'burst' mode is fast enough (probably not on the 3200), you can fire continuously as the subject gets close to the target zone. Or you can try and manually time the perfect moment. These require lots of trial and error. One of the advantages of most motorsport is they do many laps - so keep trying.