An invitation to a wedding is not a quid pro quo situation: Just because you invited someone does not mean they owe you a gift. Nonetheless, when no wedding gift is received from a guest, it's an awkward (not to mention hurtful) situation. Giving a wedding gift is the right thing to do (although there are some exceptions), it's in good form, and the vast majority of wedding guests will give a gift—but some won't. Here's exactly how to deal with wedding guests who failed to give a gift.
Month 1: Wait It Out
Wedding gifts will continue to trickle in after the wedding, especially within the first month. Beyond that, it's unlikely but it is possible (after all, tradition has long held that wedding guests have up to a year to give you a gift).
Month 2-3: Send a Thank You Note for Their Attendance
Sometimes wedding gifts are lost, misplaced, delivered incorrectly, or even stolen—and a month out from the wedding, you're probably starting to wonder if any of the missing gifts fall into those categories. The best thing to do, then, is to send a thank you card to the wedding guests who didn't give a gift and thank them for their presence at the wedding. Personalize it and make it very specific (there is nothing worse than a generic thank you note). If you're expressing your gratitude and mention of a gift is entirely missing, an astute guest who did, in fact, give you a gift will realize you didn't receive it. That's their cue to follow up with a "Hey, did you get my gift?" message, and the two of you can sort out what happened from there. And, in the event that the guest really didn't give a gift, your thank you note will still be a gracious and polite gesture.
Month 4 and Beyond: Let It Go
Chances are, if you haven't gotten a wedding gift by month four, you're not going to get one. At this point, it's pretty much safe to say the wedding guests who didn't give you a gift just aren't going to. While, technically, you could bring it up with them, I wouldn't. The bottom line is this: you are not owed a gift. Your only recourse is to let it go.
Never ask a guest why they didn't give a gift. Even though it's impolite of the guest to not give a gift, it would also be impolite to question why they didn't give one.
Special Circumstance: A Bridesmaid Didn't Give You a Wedding Gift
Sometimes bridesmaids feel that the money they've spent on the adventure so far—the bridesmaid dress, the bachelorette party, the bridal shower, etc.—all start to add up. Some bridal party members can't afford another expense, which is totally understandable. Ideally, they can approach you to tell you that, and you'll, of course, understand and tell them their presence and support is present enough.
On the other hand, however, there are bridesmaids that just don't give a gift and don't say anything—which leaves you thinking they just don't want to or don't care, which can be annoying, awkward, and, of course, hurtful. Nonetheless, the same etiquette applies: You can't question why someone didn't give a wedding gift.