For most people, expressing appreciation is a natural response to receiving a gift or accepting a kind gesture or compliment. In the business arena, acknowledging someone's efforts with a spoken: "Thanks for picking up the drawings on the way to work" is not only expected but an integral part of building goodwill. Unfortunately, there are those who have difficulty displaying their gratitude for one reason or another. They remain mute, leaving the contributor feeling perplexed and under-appreciated.
Here are a few possibilities behind their awkward silence:
They are uncomfortable. What could be perceived as a lack of interpersonal skills is sometimes a façade for social anxiety. They have difficulty communicating their emotions and subsequently hesitate to respond. Unsure of what to say, they say nothing at all. Or, they have misinterpreted the gift to be a bribe and are contemplating what step to take next. You are oblivious to their thought process and are left to wonder what's going on.
They are distracted. You were certain your colleague would love the Belgian chocolates you placed on her desk but surprisingly, you haven't heard an "ooh" or "aah" all day. What you may not realize is that she has been working late nights for several days on an audit and is preoccupied with the deadline. Between her shortage of sleep and temporary anxiety, she pushed the box of sweets under her desk, and now it has slipped her mind. Her oversight is unintentional, and she will feel terrible when she remembers she forgot to mention it.
They don't realize your effort. You transported back a fragile souvenir on the airplane, in your carry-on bag so that it wouldn't break. Your coworker thinks it is something you ordered in bulk to give out as party favors! While admittedly it is impolite to ignore your herculean efforts, they clearly aren't placing the same significance on the token as you. Yes, even the smallest labor requires acknowledgment -- and this one was no small feat!
There is unresolved conflict. When someone feels hostility towards another person, it is difficult to communicate in a positive manner. They believe that acknowledging the other party for any positive act would absolve them from the perceived offense. In their eyes, this person has been disqualified from the general niceties offered to others. If there is an issue, it is better to get it out in the open rather than harbor a grudge. Lacking the capacity to thank someone for their thoughtfulness will breed additional discord.
Regardless of the reason, there is never a valid excuse to avoid a "thank you." Intentionally doing so is dismissive and disrespectful. Keeping quiet sends the message of indifference, which can strain both social and workplace relationships. Use your best judgment when selecting your method of delivery but don't avoid those two magic words!