If you have tried caulking, and not had much success then you may consider applying a layer of concrete caulk to the problem, as this will create a better seal. Before you apply any caulking to your foundation, you should check the extent and depth of the crack. If the cracks seem very large, very deep, or are wider than one quarter of an inch, then you should take the advice of a construction expert before proceeding.
Caulk can be applied over the top of small cracks using a putty knife. Smearing a small amount over the crack will do enough to form a seal and keep the foundations dry, which will help to prevent water damage, or widening of the crack. If you have a larger gap in your wall, then you should use a screwdriver with a flat-point bit, and chip out concrete around the edges of the crack. This will help to ensure that your caulk fixes itself to the gap fully. You can also use this method to dispose of any concrete which is already loosened, or damaged by the crack.
Once you have removed the excess concrete, take a small brush and remove any loose grit and dirt from inside the crack, and then either hose or spray water into the gap. This will help to ensure that the caulk has a firm foundation with which to bond. You should then prepare the concrete caulking. The latter usually has a latex addition which helps to keep the caulk pliable, and will prevent the concrete from expanding and contracting, which is a common cause of concrete failure when it has been applied to gaps. Place the mixture into your caulking gun, and then fill the crack. Don't just apply a surface layer of caulk, but ensure that it does very deep. Push the solution deep into the crack with the gun, and then allow it to expand so that it fills the entire gap. When the caulk has finished expanding, place a little onto a putty knife or trowel, and wipe it across the edge of the crack.
After Applying Caulk
Sometimes the small crack in one part of your foundations hides bigger problems. If, after you have applied the caulk, you find water damage around the edge of the caulking, or you see that the gap has continued to expand, you should call in a professional to examine the real reason the caulk is not taking. Check your caulking regularly for signs that there are other problems with the foundation, for example swelling or shrinking of the caulk after drying. If the caulk falls out, then you should consider using a sand-and-concrete filler, rather than concrete caulk, but this will not last forever, and you may need to replace it every two years or so.