Hello @Paul Monaghan and welcome to Microsoft Q&A.
There are a few points to check before panicking. There could be a small silly thing like forgetting to reload the browser page after importing. The UI only check for new stuff on refresh.
First: Look inside the ARM template. Do you see the pipelines you expect to find?
Next: Do pipelines of the same name exist in the destination factory? They might just be replaced, rather than creating new ones.
Is there Git / repository enabled on the destination factory? Switch to Data Factory mode. Maybe the pipelines were migrated, but not ingested by Git. Git mode only looks in the repository. Data Factory mode looks at what has been published.