It is likely that the four destinations are on restricted edges. You could bring up the properties of the OD layer and on the Network Locations tab at the very bottom turn on the "Exclude restricted portions of the network". Click OK. Now, in the NA Window, right-click the Destination and choose Recalculate Location fields and then choose unlocated ( I am assuming that you have four destinations with this status). If not, you could choose All.
Re-solve. If it still cannot find paths to these locations, try turning of all the restrictions and see if that will work.
Jay Sandhu