I would completely break it down, soak, clean, LIGHTLY lubricate all internal parts and reassemble it. Simply spraying degreaser or cleaner inside, blowing it out and lubricating will not totally remove heavy, built up residue and gunk (although it is not a bad idea to do so on occasion)...guns just need to be stripped bare and totally cleaned on occasion.
If you aren't comfortable doing the job take it to a gunsmith for a cleaning job and inspection. Chances are it is just a buildup of residue but you may have a piece that needs to be repaired or replaced.
"Shoot low boys...they're riding shetland ponies in search of true grit" Lewis Grizzard