I use embossing powder MANY years older than 4 and the only kind I have age-problems with is the Sticky Stuff, because all adhesives deteriorate faster than our other supplies.
When ink I emboss doesn't shine, it most often is because I need to reink the pad I used so the ink is juicy enough to hold enough powder. This also happens to me when my VersaMark pad gets old and can't seem to absorb any more of the reinker. Or when I've held the heat on so long, the cardstock absorbed the shine. Or when I'm using SU Whisper White craft ink, which seems to have a very short window between melting to a smooth shine and melting right into the cardstock.
Some brands of ink, cardstock, embossing powder and heat guns require more or less heat, ink or powder to achieve a nice smooth shine, and there are some combinations that won't produce a smooth shine no matter what.
Bottomline, embossing isn't a one-size-fits all proposition, but ALWAYS do what others have mentioned.......let the heat gun warm up before hitting the cardstock with it, so you can get the melt to happen as quickly as possible without burning the cardstock and/or the embossing powder.
When you get a nice smooth perfectly embossed image,you can't beat the feeling that you just performed magic. LOVE that!