Why can't my rear shifter shift into low gears? I have an entry-level Trek mountain bike bought several years ago, with a Shimano derailleur. It's been hanging in my garage for the last few years, taken down only a few times for rides. I just did a ride with some hills, when I noticed it wouldn't shift into lower gears. It feels like there's so much tension in the cable that I can't physically move the shifter below gear 5.    

After spending hours this afternoon looking at bike repair videos and questions here trying to isolate the issue, I've found or done the following:  


the derailleur hanger is straight
I can manually move the derailleur to the lowest position and it shifts down to the lowest gear fine
I've cleaned out the shifter with WD40, and when I relieved the tension in the cable by taking it out of the little cable holders between the derailleur and shifter (sorry don't know correct terms); with the tension gone, the shifter moved easily between gears 1-8  
when I had the cable unclipped as above so it was loose, I was able to move the cable housing up and down the cable easily; it didn't seem dirty or much friction, although I noticed the rear gears and derailleur were pretty dirty


It hasn't had a tune up or any self-service in years. It just seems like there's so much tension in the cable that's preventing it from shifting down below gear 5. Also I noticed even when I can shift down from 8 thru 5 that the derailleur looks like it's barely moving. This is just when it's sitting still, although while moving I didn't have much trouble shifting between 5-7.
It's a little unclear whether the shifter is refusing to get into the lower gears based on what the indicator is showing or if it's indicating lower gears than 5 but nothing is happening in back. Also I'm assuming by "lower" you mean larger cogs and that you don't have a Rapid Rise rear derailleur.

If the shifter is clicking from gear to gear but not moving the rear derailleur into the large cogs, it's likely you need a bunch more cable tension. This is one of the more common break-in type adjustments bikes need. There are plenty of questions here about this, but the basic process is loosen the RD cable, screw the barrel adjuster(s) on the shifter and/or RD in, and fasten the cable back so that you have all the slack taken out, barely taut, then use the barrel adjusters to fine tune from there.

If you mean that it's having a hard time getting into the small cog (the high gears), then check to make sure there's not a cable housing end hung up on either a housing stop on the frame or the where it enters the shifter or RD.

If you did happen to have Rapid Rise, either of the above problems can still occur, but the symptom that goes with each solution would be switched in terms of which end of the cassette you can't shift into.

Friction in the housings could cause this problem too but it sounds like you've eliminated that. Make sure you've checked all the RD housings - bikes can have 1, 2 or 3 of them depending on the design.