I concur with horselips. If you are having a problem with proper operation and aren't sure why, rather than cause damage, just take it to a smith.
The problem with reassembly, as I have experienced myself, is that many guns, due to spring pressure on certain moving parts, require temporary detention of said parts for proper reassembly. The AR-15 is a good example of this where the buffer detention pin is concerned. If you do it in the incorrect way or order, you can create a situation where the gun physically goes together fine, but the alignment of internal parts is incorrect.
My advice, for your next lesson in home-gun smithing, is to see if you can find a good video online or for purchase that shows you the detailed and proper disassembly, cleaning and reassembly. This is a non-paid endorsement for the On Target video series. They do a good job showing you all the parts and steps. There are a number of youtube videos that cover various guns as well and I imagine a popular model like your 870 probably has received some attention in this area.
Best of luck with your firearm. Don't be ashamed of spending some cash on good bench time with a pro, especially if they're nice enough to show you what you may have done wrong.