1. Their circumstances change so they can no longer afford it. If indeed you have major donors, always monitor their personal and business circumstances (discreetly). A business owner who loses a major client and at the same is undergoing a divorce is obviously less likely to keep donating major amounts. You can then even pre-empt a complete stop by suggesting a reduction in the donated amount
2. They feel less engaged with your club, often because they feel ignored due to lack of communication. I cannot stress strongly enough the importance of keeping everybody who donates even the smallest amount informed and engaged. At least produce regular email newsletters, exclusively for your donors, invite them to events, send them a birthday card etc. Make them feel special!
3. The constant requests for more money from the club. Yes, I know it’s so easy just asking the same people for money all the time, but if you keep doing that you will run a serious risk of ‘killing the goose… you simply have to diversify your income generation. Sorry!