This is more of a personal case, but it's probably due of your Staraptor was in battle.
From my experience, when a Pokemon is sent into a double/tripple/rotation battle, holding exp. share of coarse, that Pokemon will steal some the other Pokemon's exp. points, given that it stays in the battle. This also happens when the Pokemon holding the exp. share is switched out. KOing the opponent's Pokemon of coarse. I haven't seen that a Pokemon getting a boost from an exp. share outside these two cases.
Only items like a lucky egg are allowed to boost exp. of a Pokemon. If you want your Staraptor to "be boosted" by an exp. share, make sure that it goes into battle for exp. theft to occur. If this isn't the case I can't help.
