It's possible that you're not using the right technique in applying the cement. Like most liquid cements, Tamiya's orange (regular) and green (extra thin) capped cements are volatile solvents. Thsi means they "glue" parts together by solvent action melting the plastic slightly so that the parts adhere to each other (this is sometimes referred to as "solvent welding") If you brush the cement onto one part and press the parts together, often, the cement simply hasn't enough time to melt the plastic sufficiently and though the parts stick initially, they will quickly fall apart.
The best way to use liquid solvent type cements is to hold the parts together and apply the brush to the join between the parts. The close contact between the parts will draw the liquid cement into the joint and give the cement more "active time" between the parts to form a good bond.
Beware. Be sparing with the cement. As the cement flows along the joint easily via capillary action, it also flows along panel lines and ridges on the parts. Be aware of where your fingers are and watch out for any lines/ridges which may draw cement below your fingertips. Many a model/part has been ruined this way.
Good luck!