I suspect that there is not enough binding agent and or pressure to make the fibres "knit" together.  Paper fibres are a bit like felting fibres, they need to catch onto each other to form the bond, this is either done through the glue paste mixed in or in the case of paper casting the pressure applied.
What recipe are you using?

Shrinkage can also do that.  Are you using molds or forms of some kind?  Are you covering the outside or the inside of them?

Papier mache (PM), strips or pulp, expands when it's wet and shrinks back to 'normal' when it dries. 

This can be helpful if you're using the inside of a press mold (where you press the pulp into the interior of a mold, such as the inside of a bowl), as the pulp will pull away from the sides of the mold as it dries, assisting with the removal of the casting.

But when you're using the outside of an object, such as a bowl or bottle (etc), when the paper or pulp shrinks, something has got to give somewhere, and esp if the form is rigid, it will be the PM that gives.  Pulp can crack, paper strips can pull apart (delaminate).

Keeping water in the pulp to the minimum may help.  Other methods you might try are covering the form with the strip method of PM, letting it dry, then applying pulp that contains the minimum amount of water that you can get away with and still be able to work the pulp; covering the mold, then using a blade to make a 'cut' on each side of the PM, creating two separate sides, and letting it dry that way.  When it dries, the cut will have separated further, relieving the stress of the pulp on itself as it dries.  If that doesn't seem viable, cover the form in two entirely separate pieces from the beginning (rather than cutting).  Either way let dry, then remove the two pieces from the form and glue the edges together and cover with a thin coating of pulp to disguise the seam.

But Newmodeller is also correct that you might want to use more adhesive in your pulp.  And less water if you can.

Sue

hello

thanks a lot for you help !!

http://michmache.skyrock.com/