Game companies often disable achievement unlocking whilst offline. This is because, while offline, it is a lot easier to "cheat the system" and hack any and all achievements available to the player. The player being online is essentially one of the greatest tools they have to ensure the achievements are unlocked honestly.
Unfortunately, for most systems, this means that any progress towards achievements offline simply do not count. You should usually find the achievements can still be unlocked, whilst online. If an achievement requires "your first win", for example, you should unlock it for your first win online - regardless of the amount of wins you have had offline.
This does not apply to all games. If you complete the same requirements, online, it could be that the achievements have been "deactivated". It could also simply be that the achievements are bugged. I can personally vouch for both being a possibility on steam.
A user posted on steam forums regarding a similar issue. They were advised by the Poker Night developers that offline achievement unlocks would require them to delete their local saves, and effectively start over. They also appear to heavily blame the steam system for this, suggesting the same rule of thumb would apply to other games. It sucks, if this is the case, but if that's how they set it up, that's how they set it up.
Steam actually have a short FAQ (of sorts) on the matter, where they openly admit that the statistics system (often used to derive achievements from) is buggy. They also suggest that achievements should be unlockable, offline. In fact, they suggest that if there is a reward for an achievement, unlocking if offline will effectively forfeit all access to the reward, what so ever.