About 20 years ago i bought a can of Bullseye. Lately when using it ,it seems like not all the powder burns when firing.Has it deteriated?

Try heavier crimp.

Properly stored powder lasts a long time.

Open up a new can of pistol powder and smell it..... then take a smell of your Bullseye. Good powder has that nitrate smell and pours pefectly fine. Bad powder has a nasty, acid like smell - and clumps. This is usually a sign it got wet - probably from condensation...... once wet the sulphur reacts with the salts in the powder and forms an acid ..... since it's a celluloide based powder, kinda like wall paper paste - this gives the acid something to work on and you get that nasty side effect acid smell.

I have a can of 30 year old powder I would show off in my reloading classes. It's just as nice as when I bought it way back when.

Modern powder has an amazingly long shelf-life if stored properly. You didn't mention what load or cartridge you're dealing with. Unburned powder is usually a sign of low pressure. I get it when I do light target .357 loads that border the minimum Bullseye load for .38 Special. With some bullets, this can be helped by seating the bullets deeper or crimping. Or, for $25 you can just buy a new pound of it and see if there's still a problem. When you have doubts about components when you reload, it's always a good idea to cautious.

Your reloading using a 20 year old can of Bullseye? That's cool! If the powder is bad you should be able to tell. I had some powder go bad and it got an acrid nasty smell. On the other hand a friend gave me some Hodgdon powder from the late 40's that was fine. That Hodgdon powder was in cardboard containers.

I have noticed the same thing with Bullseye. Will leave a few kernels in the barrel, sometimes. I am still using cans I bought in '94. No problem.

After 20 years I would imagine so. The biggest problem would likely be moisture. you could try putting some desiccant packages like the ones that come in electronics, etc. and see if that helps. If not you probably need to buy new.

Not if you kept it stored in the original container and kept the lid on tight. Bullseye is not the cleanest burning powder, in fact it is one of the dirtiest.