Not to belabor the obvious, but tech traders rarely use Bollinger Bands alone; they are always combined with another indicator, such as MACD or RSI.
I have had good (non ML) results from trading Bollinger Bands when a touch on the low band was confirmed by RSI signalling an oversold condition.
The other issue I expect you’re running into is that stocks don’t trade in a vacuum; they are largely (some more, some less) correlated with the overall market. A stock that signals BUY on a particular day will generally lose if the market declines on that day; a SELL signal will often be costly if the market advances on the day of the sale. So, somehow, the S&P index must also be modeled along with the individual stock.
But, the conclusion is the same. The market is a complex multivariate system which moves according to both rational and irrational conditions. It’s the irrational part that generally gives rational models problems. :-)
(Good example is that oil stocks will generally decline on Middle East instability, even though that instability causes spot crude prices to advance.)