Yes - by default, Bpipe restricts how many jobs it will submit. It's designed mainly as a protection against accidentally overwhelming the system Bpipe is running on. So it will not use more total threads than it thinks the computer it is running on has. For example, if it is running on a computer with 8 cores, it might limit the total cores to that. If your jobs both used for threads / cores it might only submit two jobs.
You can raise that limit by adding the -n flag:
bpipe run -n 32 pipeline.groovy ....
Note that this can also be controlled in some other ways (for example, setting concurrency in a bpipe.config file), so if that doesn't explain it you could dig a bit deeper into those other options.
Hope this helps & let me know if it's not the answer in your case!
Cheers,
Simon