what cause slow hair growth after transplantation?

You need to follow up with your doctor if you believe your hair transplant was not a success.  It is always a good idea to return for a recheck.

You will want to see your physician for advice and further guidance. In general, I recommend that anyone with poor growth post transplant should have a scalp biopsy and blood work as well as a full clinical examination. There are several possible causes. 

This is highly unusual. Your donor area needs to be examined for miniaturization. IF you had high miniaturization in your donor area, then this could be a cause of your problem as these miniaturized hair just will not grow well. 

The growth rate of your transplanted hair should be the same as your native hair.  If you gave any concerns you should do a follow,up visit with your doctor. 

You need to see a hair specialist who might be able to prescribe medication and do some tests that might give you an answer.