Some hunting team-mates and I run marlin semis very successfully. From our experience here's a guideline that may help solving most marlin semi problems. Guidelines include good advice we have received from Big Shrek, Eric0424, Arrowdodger and others on this site. Guidelines won't be 100% but I hope they are useful:
If you have a bent recoil spring and can't immediately replace it, straighten it best you can with needle nose pliers - usually still works well. Then when refitting bolt ensure spring is compressed inside bolt hole as you fit this into reciever. This saves bending springs further. Oil this spring if needed.
Note recoil spring affects bolt movement and reloading only - not firing pin/striking power. Light striking is generally speaking due to worn firing pin or worn or shortened hammer springs. Gunsmiths have often clipped a coil or two off hammer springs to help lighten triggers but it can end up causing light strikes. See FTF below.
If you have intermittent fail to cycle with your semi, determine if it is fail to load (FTL), fail to fire (FTF), or fail to eject (FTE).
* give rifle action a thorough clean and dissolve powder residue from parts especially feedthroat.
* Clean out extractor indents either side of chamber
* Check for full bolt movement
* Check for burrs at chamber entrance
* Check for worn and weak recoil spring. Replace as needed.
* Clean off all excess oil from action
* If a magazine model, slide off bottom plate and clean out magazine with white spirit/fuelite periodically to ensure bridge in mag is lifting smoothly (not jerky), and rounds coming up correctly. You can also take bottom off mags to sand edges of mag bridge a wee bit for optimum movement, but make sure you put spring back in right way. Keep mag lubed.
* If a tube model, keep tube lubed.
If you have FTF:
* may be bad cartridges with primer gel missing round bottom edge of case - common with powerpoints at the moment - or if you're carrying rounds loose in gun pouch.
* Can be worn down or damaged firing pin due to dry firing. Take pin out and check out - should have a chiselled end. Check for consistent strike depth on cartridge cases. New pin if needed.
* Could be light striking which is normally worn hammer spring due to age, or people taking 1-2 coils off to lighten trigger, or leaving action open and cocked in cupboard weakening spring.
* Could also be wear in small slot where a pin holds the firing pin into bolt.
* Occasionally new semis have a recoil spring that is too strong and will not cycle lower powered subs reliably. Possible solutions: swap in an older spring, or run in with CCI minimag hv for a brick or two, or leave action open for few days or week to soften spring - then repeat this as necessary till it cycles the subs well. See Frosty's comments in post 7 below.
* Clean action thoroughly and on reassembly put max one drop of oil on bolt surfaces only. They don't operate oiled up. Or use a proven dry lube.
* Check position of ejector wire - see rimfirecentral marlin semi sticky:
* Strip bolt and clean and check condition of extractors. Replace if needed.
* all semis are picky and will cycle some rounds well and others poorly. Marlin semis generally like all things CCI, and some also like Winchester HV. For range accuracy CCI standard Velocity solids and CCI subs usually shooot very well.
* marlin semis must be run clean and dry. Don't spray oil everywhere - that kills them. Dirty actions and excess of oil is main cause of problems. There are a couple of points in action which need occasional dot of oil or grease - go to Marlin Firearms site and download rifle manual.
* don't generally leave your action open when rifle in cupboard. Wears out recoil spring, and can weaken hammer spring if left cocked - and this causes light striking.
* don't overtighten front bedding screw - but experiment with different pressures to improve accuracy
* don't overtighten rear of trigger guard screw - breaks with too much pressure and when over-tightened can pressure receiver which may hinder proper cycling
* If trigger guard breaks - buy new or take to auto repair shop for plastic welding - cheap and very effective solution.
These basic guidelines solve most marlin problems - they are very reliable rifles with a little set up prep and maintenance.