Let's say I open a bull call spread by buying a $50 call for $3 and selling a $55 for $2. If the stock price falls to $45 for example, why can't I buy back the $55 call I sold for cheaper and make that profit? For example, if the $55 call is now worth $.50, why can't I buy it back and make $1.50 on that and then simply lose the net debit I paid for the spread?