First of all, the refi business has fallen off drastically with the latest round of interest rate increases.  So, there's just not as much business out there.  Secondly, if you want to work for a signing service (and I don't know why you would, they are nothing but vendor management companies....) then you need to sign up with the ones currently getting the work in your area.  And this is very difficult information to find, as the SS/VM doesn't show up on the paperwork trail.  Probably, the answer is that you need to sign up with most or all of them.
However, before you sign up with any of them, check their rates.  Good title companies will pay typically $150 to $300 for a signing.  Some of the SS/VM's will ask you repeatedly to do a closing for $40 or $50.  Had one recently offer me a max of $75 for a closing which was a two hour drive away.
And to really find out what a SS/VM will pay, you need to do a few signings for them, but also raise your price enough that you lose a percentage of the offers.  If you don't ever turn down work, you'll never find out what they really will pay.