Keg not carbonating, never had this problem! Does beer gas prevent co2 carbonation?

My fellow brewers I come to you in my darkest hour of desperation and confusion. I am no professional but have about a decade of brewing experience with over half of that spent kegging, I am guessing I have sucessfully kegged well over 100 cornies in that time, until now! I brewed a Caramel Amber Ale recipe that i found in the wonderful HBT recipe forum. Long story short the beer while tasty was lighter in body and much drier than I wanted. After a little research on post fermentation body building and sweetening i boiled up some lactose and racked the beer onto a 1/2 pound dissoled in a pint of water per 5 gal during the kegging process. I had some beer gas (70% Nitrogen 30% CO2) laying around for my nitro tap and used it to purge the keg of air after filling. I am pretty sure I opened the relief valve to make sure there was no pressure on the beer from the gas, then It sat in my garage at about 50 degrees for a week not connected to my gas. Next i put it in my beer fridge and had a taste before carbing. Pre-carbing tate test yielded a sweeter and fuller bodied beer

Then I did what i usually do and hooked that keg up to my CO2 in my kegorator which is an old fridge. i keep it around 40-42F and the CO2 set at 25 psi for about 36 hours. Then i lower the pressure down to my dispensung pressure of 8-10 psi, purge my keg to faucet line and take a taste. If i want more carbonation i raise the pressure up for another 1/2 day and check it again and repeat as needed. Like I said this has always worked great for me.

Well this amber ale had no carbonation at all after 4 days. So i did some research thought i may have over carbed. Vented the keg completely and repressuized to 25 and left it for several days with no luck again. I have had it at 25 psi for 5 days and not even a bubble. Does the post fermentation affect a beers ability to be force carbonated? Or does beer gas exposure affect a beers ability to be carbonated with CO2?

From what I've read, you carbonate to your CO2 volumes with CO2, then change over to the beer gas for serving. You'll need higher pressure levels, of the mixed gas, in order to maintain the carbonation level. I've not used beer gas yet, since I don't have a stout tap.

Thanks gold, thinking about this conundrum some more i recall a few years back i had run out of co2 to push my beer and used my beer gas. Back then i didnt leave my co2 or gas hooked up all the time unless carbonating. It was an older slower moving keg that eventually lost carbonation. My beer seemed like once it went flat after the beer gas exposure, i was unable to get it to carbonate again. My friend i was brewing with at the time had a similar experience but i haven't been able to confirm this with any science or from other brewers