There are a number of rooting hormones.... all are about the same but you MUST follow the directions. all they do is speed up the formation of roots.
Must also provide correct temperature range. soil moisture, & humidity for each genus/species.... its not impossible since I do it all the time....

Which cuttings did you try with no success?

As george says cuttings are not difficult. You must know someone who is regularly starting cuttings you can visit to see what you are doing wrong. Al

I don't know anyone that even grows flowers much less from cuttings. The nearest greenhouse (other than mine) to me is about an hours drive. I took cuttings of various things that had new growth and put in cloner but so far nothing has happened. If I ever get this figured out I am gonna celebrate.

When I first started using mine I had it on the bench in my greenhouse. All my cuttings started wilting after a few days. Since the cloner was black the water was getting way too hot during the day.
Now i put the cloner under the bench on the floor. It gets lots of indirect light and the water does not get so hot either. I've had great success with a variety of different things. Some things will root in a matter of days while others can take a month.
What types of plants have you tried?

I've also had terrible luck with shrub cuttings. But I've been reading some and they may root better at certain times of the year (spring) so I'm going to try that this year.