Persistant routes sometimes not working

This is the situation

We have a MSSQL cluster of 2 servers the SQL03A and SQL03B windows 2008 R2 enterprise on vmware 5.5

There are 2 persistant routes, they are for the backup:

10.64.67.171 255.255.255.255 10.37.4.30

10.64.67.169 255.255.255.255 10.37.4.29

Sometimes , one?s a month, the persistant route does not work and the backup wil not work.

We delete the persistant route and we make the persistant route again, than it wil work again.

What could be the problem?