How to use Fix SMD between runs

I need to use fix SMD in a loop but somehow it does not work properly, Is there any reason for that ? How can I solve it ?

There is not enough information here to make any assessment of the situation you are in and thus give meaningful advice.

If a command does not operate as documented, you have to provide proof!
“Does not work properly” is a very unscientific description that neither mentions how you are using the command nor what kind of behavior you expect. Obviously, one cannot give a reason or a suggestion for a solution for something that is mentioned only in this vague fashion.

Please have a look at the “guidelines” post to have a better chance for a meaningful and on-topic response.