NMM has a major bug where if mods are stored on a drive other than the drive where the game is installed, it will have incomplete or failed installs a lot of the time. I frankly do not understand why this has never been fixed, but it's a common issue.  Please read this in full for solutions.

This happens not due to a bug, it is simply because the texture or material assets are not loading due to something going wrong with the configuration, or install process.  Literally 100% of reports have been due to something going wrong with the install, the configs, or something else on the user side and about 90% of the time it is the fault of NMM.

• The files are not loading correctly.  NMM can sometimes cause problems with its virtual directories if you are storing mods on a separate drive than your drive where Fallout 4 is installed.  It can also happen if you are using Profiles functionality.
  
  As one user stated, making sure the virtual folder is set to the same drive as the Fallout 4 install can help the problem:
  
  "Just fixed my problem - I have different drives and under the NMM "settings" everything was on the same drive as my game. Eventually I realized the virtual folder info is under the NMM "tools". Once I changed that to the same drive as the game everything was solved."
  
  Other users reported that running NMM as administrator fixed it for them:
  
  "Try run nexus mod as administrator, then disable and enable mod. I had same problem and now it's working"
  
  If your INI and NMM settings are absolutely correct, and you're still experiencing blocky rain, you may be a candidate for a manual install.