I did this recently and I discovered that the address of the preview file was wrong slightly. Can also happen if the preview file extension is wrong e.g. bmp instead of jpg
5 x WCC - S5, S6, S7, S8, S10 | 4 x WDC - S5 Nick van der Voort ; S6, S7, S8 Ed Greenhalgh | 2 x WTC - S6 Stuart Ingers; S8 Andrei Sevastian