Making old posts show in RSS Feed

I have a blog (gospelebooks.net) that posts daily deals for Kindle books. Often times a deal expires and then many months later it becomes a deal again. Instead of re-making the post I just go back to the old one and set the timestamp as todays date-time and it displays on the homepage. The only problem is I just found out my RSS subscribers do not get it show up in their feed. How can I correct this?