Some calls work, some don''t

I use wildfire server, asteriskwin32, spark, and softphone(expresstalk).

Then I call from spark to softphone, some work and some don’'t.

When I call to one user then I call to the same user again, the later call didn’'t work if i call immediately after hangup the previous one.

How long does it take before i can make another call?

When I use only spark(not use softphone), I can’'t make any call.

Do i need other softphone besides spark?

Thanks in advance