If you are getting a good conversion of your mash, and you mash temps are per recipe, and remain there during the mash, then I think your main problem is either in the yeast or the temp of your fermentation.
Does your fermentation area keep a constant temp?
do you take the temp of your fermentation from the fermentator or the room? The temp in the fermetator will be a few degrees higher than the room temp. and fluctuating temps is not that good.
Do you use a yeast starter? and how much per 5 gallon? Are you using a yeast nutrient?
ok, how about caramelizing of the wort? that would make some sugars unfermentable.
I found that when I started using starters that it was one of the biggest steps I ever had to stabilizing fermentation of my beers. I use 2000 ml starter per 5 gallons, use a stir plate, and yeast nutrient. My beers take off fast and finish on target. The outside of my fermentor is insulated to help keep it from accidentally having temperature fluctuations. And I have an old cooler I use as a fermentation/ Lagering closet.
the biggest thing to understand is that there is both a learning curve, and equipment curve to brewing beer. The more you brew, the more you learn techniques, the more you learn, and that helps so much. But also you need to have a eye out for stuff you can turn into brewing equipment. Finding a small cooler that you can put a good controller on to use as a fermentation closet will vastly help in your fermentations. Not only that but you have a lagering closet also.
If you do not want to invest into such a closet, well remember we are all just having fun and many of us do not use them for a lot of reasons, the biggest being expense or space requirements.
and as always, keep notes on all things, no matter how minute, as you change things so you know what is working and what is not. A huge mistake made by way to many brewers is not making notes.
but the biggest thing to remember is to have fun and drink beer. As you get better, and as your beers do to, you will figure out that this is just part of the process of learning so reread this thread, they have posted a lot of good info, and start your changes as you can afford them. You can make starters without a stir plate, one step at a time.