Technically, your wife's HCG level is not high enough. Therefore
you are seeing one line only.
In layman's term, the sign of a pregnancy is weak.
Solution: test again next week using a new test kit. Of course
you can go see a doctor to confirm, only need to pay more $$$ than
a test kit.
-----
If there is pregnancy, you will see a 'plus' sign and a 'minus'
sign. (The minus sign is just to tell you the reaction has
completed, and you are ready to check if there is any 'plus'.
----------
If HCG level is high enough, you will see very bold and thick
'plus' and 'minus' signs. Congratulation, you have a very strong
fetus in womb.
If you see a faint 'plus' and 'minus' sign, congratulation too,
need take good care during the first trimester.
Good luck. Remember to take Folic acid.