Okey i have had this problem couple of times and i'm tired of it and just want to find the solution if anyone has it

I've now had like 4 computers that i played Oblivion on, but it usually goes the way "I get exited haven't played Oblivion for a long time, install, get some mods, get tired, removes" then like after a half year i want to play again then i "install, get some mods" but the mods dont work, it's like, every first time i install Oblivion the Mods work, or after i've formatted, but when i play again the mods don't work.

Now when i got this computer, like 4 months ago, i wasnt going to make the same mistake because i didnt know what was wrong, and i install the game, mods work, but i get the Construction Set and then all the mods still work, but the new mods i download dont work, just the original i had before i downloaded CS...

Has anyone experienced this? or am i one about it?