Frosting experts needed. Frosting doesn't get fluffy orthick sometimes

Your frosting recipe is a variation of buttercream frosting which can be fiddley and finicky. That relies on everything being at near the same temperature, with the butter being definitely soft to start with, and the milk at the same temperature. If anything is too warm or too cool- either the fat clumps or the liquid separates out and you get curdled looking frosting. You can fix one that has broken, by adjusting the temperature according to what you think was wrong- too warm, chill it down a bit and rewhip. To cold, then warm it up and whip again. The frosting can also break if you whip it too fast or too long- bringing in too much air to the mix. To fix that, you can try adding in some cream while very slowly beating the mixture until it reforms correctly.

If I were you, I'd warm the milk in the microwave a few seconds to warm it slightly. Then whip the oleo/butter and crisco until it's fluffy, and then add in the yolk, sugar and vanilla. Once that has mixed, add in your warmish milk while beating at medium speed until your frosting forms and is the consistency you like. That ought to get you the fluffy stuff more often. Hope it works out for you and you can continue to enjoy the family tradition.

Here is a picture of the frosting