It's a bug in the game and also some versions of franken mod if you are using that.
If you are still using franken mod you are missing out on alot of new stuff that has been added (if you are playing vanilla I think the same...but that's a different story

)
Only known way to fix this problem is to install patch-2 along with a recent version of Bman's Mod. Goto
http://www.ufo-scene.com to get info on the mod. But here are some quick directions:
0. SAVE YOUR GAME ON THE STRATEGIC MAP (while looking at the planet). The new version of the mod will not be able to load your old combat savegames. It can only load your old strategic savegames.
1. Download and install official patch-2
Make sure when you install you verify it is installing to the correct folder. The installer has a nasty bug where it usually defaults to the wrong folder and does not warn you the patch failed.
2. Download Bman Mod v4.07 here:
http://www.savefile.com/files/1185404 Just unzip the file into your UFOET\data folder, replacing the existing files. This is the main mod and the only *required* download
3. Download Bman Multimedia Pack 4.02 here:
http://ufo-scene.com/download.php?view.99 Just unzip into your UFOET\data folder, preserving folder names. You should end up with a UFOET\data\beousounds folder for example. This download has all of the sounds and ufopedia graphics the mod uses. It is a big download and not required but highly recommended.
4. Download Bman True-Night Mod Optimized EXE here:
http://ufo-scene.com/download.php?view.95 Unzip into your main UFOET folder. If you plan on enabling the True Night Mod from within the in-game mod settings, then start the game using this .EXE instead of the normal .EXE. Just make a shortcut to this exe on your desktop for easy starting. This lets you play with the true night lighting without any performance penalties. This download is optional.
5. Download Bman Enhanced CAF Sprites here:
http://ufo-scene.com/download.php?view.89 Unzip into your UFOET\data folder. This causes the graphics for your soldiers to be a bit crisper and more defined during combat. Optional download
6. Download Bman Loading Screens 4.0 here:
http://ufo-scene.com/download.php?view.87 Unzip into your UFOET\data folder. This provides a bunch of new loading and enemy movement screens. So when you are loading a map or waiting for the aliens to move, you will see alot of different "please wait" screens to make the time go by faster. Optional download
After all that, I kinda recommend starting a new game as it is alot different, though you can certainly continue your old game. Play with the in-game mod settings screen. There are *alot* of options and new features. All settings have popup descriptions to help you figure out what it does. Both the strategic and combat game is alot different.
New research - new weapons, grenades, rockets, guns, etc.
New enemies (if XCOM Alien Invasion is enabled)
New computer controlled allies (if you capture/research some of the new aliens)
New boss monster (if Boss Monster Frequency is set to allow it to occur)
UFO Attack Groups (if enabled)
UFO Swarm special events (if enabled)
New powerful special equipment that can be found after combat
If you play with UFO Attack Groups and/or UFO Swarms, I highly recommend you set the Alien Bounty to $20k so you earn $20k for each alien you kill. You *will* need the money to build enough fighters to battle the large numbers of UFO's you will encounter.