No, IVF is never 100% sure and successful. You have to well realise it before starting your treatment journey. Statistics and success rates can give you a general idea, but your individual chances always depend on your individual circumstances. IVF success is related to a number of factors, some of which you have little control over - like the cause of your infertility (e.g. poor ovarian response, poor endometrial receptivity, fibroid tumours or uterine abnormalities) or your age.
In fact, the latter is probably the most important IVF success factor. The decrease in the success rates with IVF or any infertility treatment is due to the decreasing number of eggs within the ovaries. There is an accelerated loss of the eggs after the age of 32-35 and the fertility potential is significantly decreased afterwards. There are also other factors to be taken into account, such as previous pregnancies, miscarriages or simply lifestyle habits. Surely all the causes known to affect the IVF outcome should be identified and regulated prior to initiating treatment.