I don't know the answer but I'm pretty sure your case is not unusual. I submitted the Sunday Times Crossowrd and Brainteaser most weeks for several years but never won.
There will be many thousand entries for the MOS crossword each week. I would guess at least 5000. So if you enter every week I reckon you'd be lucky to win once every hundred years.
I once read that a vicar submitted a crossword every week for over 20 years without success. He finally won, but his letter and prize arrived on the day of his funeral. Best to keep on submitting and hope that you never win !
As far as I can recall at least 25 years, sent every week including when on holiday in the UK. By the way my "pet" name by my good lady wife, would you believe is "Big Ears"
I've been doing the You Magazine crossword for at least 18 years without a win - generally lucky though, as have had runner-up wins with the Daily Record/Sunday mail - also local quiz fundraising things and have won our town magazine ( yes, other people do it too!!!)crossword several times and its NOT easy - the Big One would be nice though.
My middle daughter and I have been doing it since she was in her teens with the proviso that if one of us won we would split the winnings. She is now 37, married and still takes the Mail on Sunday but we are still waiting although I did win £250 on the You mag a couple or so years back.
One day Thomas. One day.