I use the l'oreal casting creme gloss and have done for years. But I'm 50 now and there is a LOT more grey, so it does take a lot more effort. A few tips...
1 I have to do it a lot more frequently than I used to. I'd previously dye my hair maybe once every 6-7 weeks, now i do it at least once every 4-5.
2 The grey starts to really come through at the roots, even before the colour is significantly faded. When I do my hair, I start by taking the nozzle and running it in lines from my hairline (above my forehead) back towards the middle of my head - so I'll go from the middle of my forehead doing lines very close together until I get to my ear, then back and down the other side and back. When I had less grey, I only had to do this in my parting! 
. Then I massage all that product into the roots very enthusiastically. Then I do it again, massage again. Then a third time. You can really see the difference by the third time - the hair starts to have that gloopy, covered look and the grey can't be seen through the product really.
3 Then I put it through the rest of my hair. My hair is quite short (I can't even tie it up) and I use the entire bottle of product (although I've used at least half before I even start this process already). If my hair was even a little longer, I'd have a second bottle and if it was anything longer than about shoulder length, I'd expect to use the FULL second bottle as well. Very long hair might well take 3.
4 Then again, enthusiastic rubbing in and massaging of the hair all the way down the shaft. Mine is well covered by now so I don't have to do this as obsessively as I would if I was colouring for the first time though.
By now I've probably been working on it for at least 10 minutes. Then I leave it for another 20 before I wash it out.
My hair is also very dry and course so I find I use the conditioner that comes with it. Then I rinse that. Then I use a very quick light amount of shampoo (and I use L'oreal vitamino essence shampoo for coloured hair), rinse, then apply my regular conditioner (same brand). I find this seems to make the colour come out shinier, glossier and less odd. Hard to explain!
In your case, if your greys are not well covered currently, I expect that even if you do my technique, you'd probably need to do it again a week or two later the first time, just to really get those hair follicles more covered.
Final tip that I seem to have had to do that I didn't before but that might just be my colour choices - I like a hint of red in mind but I think because of all the grey, if I use anything with red, it comes out way TOO red. So I now buy a brown and a red and I mix them together at a 1/3 red, 2/3 brown ratio. I definitely didn't have a problem with the tone before but like I said, I think it's because the grey takes on more red tones so it gets very bright and brassy too easily.