My cookies aren't shiny when they dry.  why?  I'm using thinned RI.

If you care to share the recipe you are using, squirrelly, maybe someone will be able to help.  I've rarely used royal icing, so have no ideas.

you can add gum arabic to the royal icing and they will shine -- gum arabic is found at cake deco stores -- don't know if walmart and michael's would carry it -- royal icing is not necessarily shiny it is matte when dry

https://www.amazon.com/ounces-Pure-Gum-Arabic-Powder/dp/B07YCK8VX8/ref=sr_1_16?dchild=1&keywords=gum+arabic&qid=1591051527&sr=8-16

K8s suggestion is a good one. 

Also, if you dry the royal under lamp, close-ish to the bulb --it will dry shinier.  The shine will dull again if the icing is exposed to humidity. 

I add a tap on corn syrup and glycerin to my royal icing if I want a shine. As posted, RI dries to mat finish. If you want a shine that will last for a couple of days, brush your cookies with a mixture of clear corn syrup and alcohol. 1 part corn syrup to 3 parts alcohol.

Oh, and I should have said...this recipe uses NO EGG WHITES. Here’s the recipe  link.

https://todayscreativefood.com/glossy-royal-icing/ 

https://www.foodnetwork.com/recipes/alton-brown/royal-icing-recipe-1941917

Welllll, they can call it what they want, but that's NOT royal icing----it's a glaze that's thickened with corn syrup.  Lovely alternative, but highly hygroscopic [draws water from the atmosphere easily].  It'll be very shiny when wet, but no reason for it to stay shiny--sorry.

maybenot You are correct it certainly is NOT royal icing. In all honesty the person posting the shouldn’t refer to it as RI. Perhaps another name. As you posted an alternative....maybe “Mock Royal Icing”. I actually stumbled across this recipe when I wanted something super quick to do some cookies. Made just a small batch. Don’t know how freeze worthy the cookies would be with this icing. And I don’t know about the continuous “shine”, as I only made 1/2 the recipe and cookies were eaten within hours.