A few ideas to test:

1. what settings on spatial filter do you have? https://docs.safe.com/fme/html/FME-Form-Documentation/FME-Transformers/Transformers/spatialfilter.htm# read up about the Tests>Filter Types about how to process filter vs canditate
2. add a decelerator (0.2sec) and find out where the features are accumulating.... you may want to place a featureHolder somewhere if you can't get the setting in 1. optimised

We also kind of need the settings of both your SpatialFilter and the SpatialRelator. And what do you mean with 'the filter fails'?

I tried Multiple settings in the spatial filter. Filter type = 'Multiple', Pass Criteria = 'Pass Against All Filters' and Spatial Predicates to Test = 'Within' and 'Intersects'

Also, Filter type = 'Single Filter', Pass Criteria = 'One Filter' and Spatial Predicates to Test = 'Intersects'

As for the realtor I only tried it using all of the Requestor OGC options.

When I say failed I am referring to the Passed/Failed outputs in the spatial filter transformer.

If you set your SpatialFilter to "Pass against all filters" it means that your candidate feature must have a spatial relationship (intersect or within) with all the features in the filter port. E.g:

I have set my SpatialFilter to "Filter OGC-Intersects Candidate" and "Pass Against All Filters"

Just the middle square of my Candidate features (in pink) passes, since that is the only feature that intersects all 4 Filter features (in green).

Hope this helps.

Also, have a look at this page: Spatial Relations Defined. This might help with choosing which OGC test is best for your data. For example, the "Intersects" test also includes the "Within" test.

I've got it now. I was confused by the transformer parameters. I thought all of the 'Tests' where referring to the 'Spatial Predicates to Test'. I didn't realize the 'Filter Type' and 'Pass Criteria' are referring to the input by which I am filtering the candidate.

Thank you for clearing that up for me.