Weird sparql problem

[Mikael Pesonen]

This returns one row fast, say :C1

SELECT *
FROM <https://a.b.c>
WHERE {
  <https://x.y.z> a ?t .
  #?t skos:prefLabel ?l
}


and this too:

SELECT *
FROM <https://a.b.c>
WHERE {
  #<https://x.y.z> a ?t .
  :C1 skos:prefLabel ?l
}


But this always hangs until timeout

SELECT *
FROM <https://a.b.c>
WHERE {
  <https://x.y.z> a ?t .
  ?t skos:prefLabel ?l
}

What am I missing here? I'm using Fuseki web GUI. Thanks!