Sometimes a better opportunity arises while you are already holding stocks and fully invested. Other times, those same opportunities arise and a quick portfolio adjustment is necessary.  The problem is executing the changes needed as sometimes it makes us feel wrong, dumb, desperate, or...

If you aim for higher returns you will see higher volatility within your portfolio and that's normal. You are not perfect and will not catch the perfect bottom, the stocks falling because it's distressed which is the whole reason you bought in the first place...don't act surprised. 

Growing vs Dying Companies in the Stock Market.