unwanted page break in Access report

I have a report by X. For each value of X there is the name of X followed by a continuous subform, matched by child/master to X. For some values of X there is an unspecifed page break before the subform information. I can see no pattern – this even happens for one value with an empty subform.

I have this problem with two reports, with different X and different subforms. There seems to be no parallelism between the two errors. Where should I look?