It happened to me last week when I deposited 0.06 BTC on a casino just for 100% deposit bonus promo, I played on slots there but just in a minutes lost that amount of 0.12 BTC same without winning anything there, slots always very bad to play there because players very low chance to make profit.
Which casino was this that gave you the deposit bonus right away to play with?
I thought most of these casinos only give them to you once you spend that deposit and over a certain amount of time.