Blue Tit nest dropping eggs

Hello,

We have a pair of blue tits nesting in a box in our garden. Since the start of May, we have found 1 broken egg under the nest on the ground per day (in line with laying behaviours I understand). Any idea why this may be happening?

The birds seem to be still busy building the nest with lots of moss on the ground under the box also.

Do they reject eggs?

Any information appreciated.

Thanks.