Sigma Em-140 Macro Flash help needed
I'd expect the flash is not compatible with the Pentax through the lens metering. Pentax uses a P-TTL system where there is a pre-flash where exposure is measured, followed the main flash. The time between the two flashes is quite short so it appears there is only one flash.
To test your P-TTL, use the built-in flash and install a Pentax kit lens or DA prime, and turn on the 2 second self timer. When you press the shutter release, you will see the pre-flash and then 2 seconds later you will see the second flash. Then you can see if this works with the Tamron mounted and then turn off the built-in flash and use the Sigma flash.
This should tell you what's causing the problem.