Could be a number of things like improper install, wood shrinkage, framing deflection. 

Plus we decide case by case what we feel like doing.What we do ourselves (in general) - We paint both interior and exterior, hang and tape drywall (but on the last big drywall job we hired a couple of guys to hang the ceiling pieces), we fabricate and install trim, hang interior doors, install some flooring.

For color variation I looked at the lot number on the boxes and pulled from six different lots when I installed.

@Josh Cohen - Bigger Pockets has two great books that can be found here - The Book on Estimating Rehab Costs is a fantastic source of numbers, while The Book on Flipping Houses is a fantastic source of strategy.I believe that you can get them in PDF form or hard copy from either BP or Amazon.To give you an idea of what the first book has - an example page is for flooring, where it will go over all of your options (laminate, hard wood, tile, etc) with cost per square foot, installation cost, comparisons for how long each option will last before needing to be replaced, etc.

I will stay away from the ultra cheapo stuff but want to keep material costs below $2/sqft and will install myself.

Eliminate the carpet and drapes from your units- install commercial vinyl tile (NOT the cheap-assed peel and stick type!)

And when NOTHING will work it's necessary to make sure that you kept some decent sized pieces of carpet left over from the installation.

Some great advice here.I would also like to ad that I buy full kitchens and bathrooms off craigslist and offerupnow for a fraction of the price of installing a brand new one.I only buy materials which look new.Recently I bought a kitchen with granite bench tops and all appliances for $700.

I have bush hogged a 1/2 acre of bamboo , installed lots of drainage , replaced 4 water lines to houses , tore down that building , rototilled 11 lawns , cut my lawn and field . plowed snow (great for walks) .

That being said, I'm starting to use laminate everywhere on all my newly acquired buy-and-hold properties since I can get it for 0.39-0.89/sqft and install it myself very quickly.