There reallly isn't that much inside. If you look at this diagram part 40 is the cartridge stop. It is a finger that pops un behind the cartriges to keep them from coming out of the magazine tube. When you pump to the rear a spring forces the finger up and it blocks the cartridges. When the pump is pushed forward the bolt pushes the finger down and a cartridge is released onto the carrier. Something is wrong with that finger. Might just need cleaning. Take it apart and inspect the insides. You did not mention what caliber you are shooting. If you just recently took the magazine tube out and it is a .357 did you put the mag tube reducer back in?
http://www.davide-pedersoli.com/uploads/prodotti/S-V-L_924.pdf