If you check the Reddit Wiki listing all the SpaceX Falcon 9 first stage cores, you will see that they are flying the Block 3 and Block 4 boosters only twice.
What is interesting has been that they still often fly them on the second flight with legs.
They have been using these flights to test the edges of the flight envelope. In some ways, the Block 3/4 boosters were a development and test program for the reusability process. The lessons learned (Per Musk, about 100 changes) were collected and rolled into the Block 5.
I heard, but cannot find the quote that they think they could fly a Block 4 more times, but that it would cost too much to refurbish, especially as the Block 5's, meant for easy reuse and minimal refurb between flights is now rolling off the production line full steam.
The Iridium-6 launch, used the core from Zuma (Core 1043), the SES-12 launch due after is using the X37B core (1040), the CRS-15 launch has a core 1045 from the TESS mission, and then they are out of Block 4's and it is Block 5 all the way down to the turtles.
Now as it turns out, I expect that getting Block 5 ready for reflight will cause a delay in their process flow. They have said they want to disassemble the Banaganbhu core (1046) to examine it to see how well the changes worked. This will take some amount of time. Currently only 1047 has been spotted (1048 might have just been spotted at McGregor) and they only build about 10-12 first stages a year.
If you check out SpaceX Stats, about midway down the page they list the Reflights/Reuses statistics. One of them is time between flights.
Used to be 160+ days for the quickest reuse, as I look now, it is 134 days.
The 1045 core, the last new Block 4 was flown on TESS, Apr 27, 2018 and is scheduled to fly Jun 28, 2018, which would knock that number down to about 60 days or so. But to keep up the two week cadence they need more cores or reflyable cores, and if they are delayed looking at 1046's first flight, this could hold up the cadence of clearing the manifest.