Any reason as to why am I unable to flash a previous firmware?

I was tinkering around with firmwares the past few days because I was getting tired of my signal dropping and having to manually do the "*#*#4636#*#*" method or turning airplane mode on and off to get signal back only temporarily. I rooted my SM-N910P phone with the 5.0.1 firmware but the CM12.1 nightlies I got were based on 5.1.1. I managed to successfully flash the 5.1.1 firmware through ODIN which meant I lost my rooting capabilities.

I already made the mistake of trying to get root access back by reflashing the cf auto root via ODIN which was based on 4.4.4 but ended up just getting in a bootloop. So I had to reflash the 5.1.1 firmware which got rid of the bootloop. But now that I have it on my phone, I am unable to flash any previous STOCK firmwares. I've tried both 5.0.1 and 4.4.4 firmwares on the Sammobile website and end up getting a "FAIL!" message despite my phone being recognized on ODIN and ODIN reading the tar md5 firmware successfully under AP/PDA.

Code:

---

<ID:0/005> Added!!
<OSM> Enter CS for MD5..
<OSM> Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> N910PVPU3BOF5_N910PSPT3BOF5_N910PVPU3BOF5_HOME.tar.md5 is valid.
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/005> Odin v.3 engine (ID:5)..
<ID:0/005> File analysis..
<ID:0/005> SetupConnection..
<ID:0/005> Initialzation..
<ID:0/005> Get PIT for mapping..
<ID:0/005> Firmware update start..
<ID:0/005> aboot.mbn
<ID:0/005> NAND Write Start!!
<ID:0/005> sbl1.mbn
<ID:0/005> rpm.mbn
<ID:0/005> FAIL! (Auth)
<OSM> All threads completed. (succeed 0 / failed 1)

---

I've done a master/factory reset from recovery and it didn't help as I am still unable to flash a previous stock firmware. Since I already made the mistake of flashing two incompatible files that led to the bootloop, I was hoping to just go back to the 5.0.1 firmware I downloaded from Sammobile then just getting the last updated version of CM12 based on 5.0.1.

Is there something about the 5.1.1 firmware that prevents this? If so, I think I'm just gonna stay unrooted for now until there is a viable rooting method for 5.1.1.

Current 5.1.1 firmware for the SM-N910P version: N910PVPU4COG5_N910PSPT4COG5_N910PVPU4COG5_HOME.tar.md5
5.0.1 firmware from Sammobile I've tried to flash: N910PVPU3BOF5_N910PSPT3BOF5_N910PVPU3BOF5_HOME.tar.md5

Any feedback will be very much appreciated!