Why KMPlayer Doesn’t Play MP4, FLV, MKV Files In order to fix ‘KMPlayer doesnt work’ problem, please take a closer look at the following help and 2 tips to help you out! As most players shouldn’t have problems with MP4 and H.264, KMPlayer should play both, as well as MKV, AVI, FLV, WMV and AVCHD.