OK, have you thought about taking the wooden dowel (plug) out to see if it had slipped between the spring spring coil? That seems like the easiest way to check it would be to remove it altogether.
If the spring itself is kinked or past its' service life, you could order a replacement spring from Mossberg. Or even midwayusa has them in stock for $5.
http://www.midwayusa.com/product/936744/mossberg-magazine-tube-spring-mossberg-500-20-12-gauge
If the spring itself is kinked or past its' service life, you could order a replacement spring from Mossberg. Or even midwayusa has them in stock for $5.
http://www.midwayusa.com/product/936744/mossberg-magazine-tube-spring-mossberg-500-20-12-gauge