A bugaboo brake fault can be caused by any one of the three parts that make up the brake system.
1/ The cable and cable adjustment screw.
2/ The brake lever assembly.
3/ The brake shoe assembly.
I've listed the three parts in the order of most likely to cause your brake problem. First I'll give a quick over view of how the brake should work then we'll look at the 3 parts in more detail.
Lifting the brake lever up pulls up on the brake cable, a ratchet in the lever keeps any tension until you press the release button in the centre of the lever. The cable pulls up on a toothed brake shoe and via a connecting axle a matching brake shoe on the other side, these brake shoes engage a set of matching teeth on the inside rims of the rear wheels thus locking the wheels.
A spring on the cable side brake shoe disengages the brake shoes when the release button disengages the ratchet in the brake lever.
This is a simple system with most parts in plain view so you can see the operation of all parts except the ratchet inside the lever.
1/ The brake cable is the most likely cause of most brake problems.
Like all cables they can stretch, stick and snap.
If the brake lever seems to work OK but the breaks don't hold very well try adjusting the cable tension by winding out the cable adjustment screw at the bottom of the cable thus taking up any slack and bringing the brake shoes up tighter into the wheel rims.
A sticky or broken cable should be obvious by observing the brake action as you pull and release the brake lever.
Run some sewing machine or 3 in 1 oil down the inner cable wire if it's sticky.
If broken you can make a new cable yourself. You will need a mountain bike cable a solder less nipple and 75 cm of outer cable if yours is damaged. My local bike shop sells these parts for $10.50 they have to order in the nipples. You can reuse the cable adjuster my local bike shop can get them for a $1 each!
The Bugaboo has an odd double nipple design. Your replacement cable will only have one end the same as the Bugaboo. The other end will be either bare or have the pear style nipple. This you would cut off. Using the old cable as your guide, you cut the new outer cable to 75 cm and fit the ferrules supplied with the kit. The matching nipple will be the top (lever) end of the cable, reuse the old adjuster and slip it on to the bottom end followed by the solder less nipple. Set the nipple 89 cm from its opposite. Depending on the cable and nipple you buy you may need to drill the plastic parts out to ease the fit I also cut the screw head off to get a better finish some nipples come with internal grub screws.
If this sounds to complicated just print this out and give it to your local bike shop. Here's one I made all ready to go! 
2/ The brake lever assembly is made up of the lever, a release button, a small compression spring and the outer bracket that holds it all on the handle.
The bracket also holds the left handle release button.
The most common problem is the ratchet fails to hold and this is a safety issue as it can fail quite suddenly!
Before reading any further have you ever lubricated you brake lever? If not do so I have quite a few brake lever faults come to me that are just sticky buttons. Use a silicone spray not WD40. Now back to real brake problems.
Either the small compression spring has broken (unlikely) the screws holding the assembly loosen (hopefully) or the teeth on the button and outer bracket have worn down (most likely).
Try first to tighten the two screws, if that does not help take the two screws out.
It's a simple mechanism the button sits inside the lever it can move in and out of the lever but can't turn as it has splines that lock it to the lever.
A compression spring pushes the top outer edge of the button against the inner side of the bracket; they both have serrated teeth that lock them together. The raised centre of the button protrudes though the bracket.
Pushing on the center of the button disengages the interlocking teeth of the ratchet. The compression spring can be found in most hardware stores, worn ratchet teeth can be reshaped with a rotary modeling tool like the Demel.
3/ If the Brake shoes are the problem it will be obvious,broken expansion springs are easily replaced, stripped threads on the cable adjuster can be sorted at your local bike shop and broken parts either repaired by a plastic welder / bumper repair shop or replaced with parts from a salvaged frame.
The most common problem is one shoe engaging before the other (usually the one opposite the brake lever) this is caused by the aluminum connecting shaft getting a slight twist in it. Remove the wheels, take the screw out of the connecting shaft of the shoe that engages first. Lever the shoe out of the aluminum shaft with a large screw driver. Chisel the alignment splines off. refit shoe but don't put the screw in yet. Refit wheels, now put the brake on and check that both wheels now lock up. While brake is still on fit screw back drilling a new hole if necessary, Release brake and adjust brake cable so break shoes just clear the wheel rims, Check brake operation if you find you cant get the lever to a suitable lock position back the cable off a bit more and try again,
