What do you mean by "flow back"?
Are you getting some blood flowing out of the injection site after you pull the needle out?
If so, it's not a huge concern as it can happen occasionally.
I like the "Z Track" method of injecting IM and have been doing it for years with much success
Simply pull the skin at the site of the IM injection. When it's pulled tight, inject the needle and press the plunger as you normally would do.
When you are done, pull the needle out and release the pulled skin.
What happens with this method is that the hole on the skin is now away from the injection site so the likelihood of any blood or testosterone leaking out of the hole is much less.