Yes could be that your DS300 not send data, to know just go in the DS300 interface of Pc Lap Counter and click on the debug button, this will display the data from the DS300.
Also some DS300 work at 4800 baud and other at 56000 baud, i suggest you try the different baud.
For your information each time you launch the Pc Lap Counter program it is necessary to synchronize Pc Lap Counter with the DS, to synchronize simple push the button start/stop of the DS lap counter or let a car pass the detector.
Synchronization is successful when the light in the DS interface of Pc Lap Counter is GREEN.
It is not necessary to make this handling at each race or segment, only after the launching of the Pc Lap Counter program.
If it's not still working then i suggest you test your DS300 with another software and see what happen and if you have the same problem with other software then i'am afraid that your DS300 is broken, i had already users with bad DS lap counter and the solution was to replace the chip ST 232 CN of the DS lap counter.
pclapcounter video channel