First time carbing and problems

I just kegged up some brews and used the force carbonation method. I let the beer get down to 40 degrees (my fridge was at that temp) and then blasted them with 30psi and shook for a minute half. I then placed them back into the fridge and kept the pressure at 30. After a day i turned down the pressure to 15 and purged the kegs a couple times each to release any oxygen that was present. I should add i did this a couple times after shaking them. I turned down the psi to 6 to serve and all i get is foam and a little liquid. The glass starts to trun to beer after a while. WHen held up to light there seems to be little to no bubbles in the beer. when i drink it it taste flat and there are bubbles present but only on the top and there seems to be many bubbles! What am i doing wrong or what can i do better. Ive read many of the stickys and many other websites and thats where i got my carbing technique. Am i doing that wrong? My beers are both wheat beers and the fridge is at 40 degrees using co2 air line about 4 feet and the beer line about 3 feet. I dont know what other info would be needed

Lots of ways to do this

We hit the keg with 40 psi for 3-4 days checking every day and adding more to keep the keg at 40 PSI

Then when you want to serve you dump the pressure or most of it until you cant
get any beer. then keep hitting the keg with 2-3 psi at a time until you get a good result.