It’s a little more nuanced than that.
Right now there are two rule engines, Rules DSL Engine and NGRE.
Rules DSL Engine:
- only run Rules DSL Rules
- has a limit of only five Rules that can run at the same time (the likely cause of OP’s problem)
- does not have a REST API (i.e. doesn’t appear in PaperUI or what will be it’s replacement)
- in OH 3 will also support Rules DSL (seems to be the plan at least)
- supports building rules using text source files or building rules through the REST API (i.e. through a GUI)
- unlike Rules DSL Engine, will only run one instance of a given rule at a time
- unlike Rules DSL Engine, there is no limit to the number of different rules that can run simultaneously
Rules DSL Engine is likely going away. Rules DSL as a language to write rules itself is likely going to be around for a bit pending overcoming technical difficulties.
As with all things OH 3, stuff is in flux and is likely to change several times before the first beta release.
As Andrew pointed out with the link to that thread, Rules DSL Engine can only run up to five Rules at the same time. If you have long running Rules (anything more than a couple hundred milliseconds is considered long running) you may encounter a situation where all five threads are in use meaning that later rule triggers either have to wait or get dropped.
This is a limitation of the Rules DSL Engine and, as mentioned, that engine is going away. This is not a problem with NGRE and it shouldn’t be a problem with NGRE even when using Rules DSL. Though I can’t say that for sure.