Does this problem happen on some specific routers of the 30 or randomly on any few.

You can try the below and update as to what is the effect.

1) Try increasing the interpacket delay for RIP updates. By default its none, but you may want to introduce a delay just in case to check if the packets being dropped is the reason for some routers not reflecting the updates.

Router(config)# router rip

Router(router-config)#output-delay 15

2) Alternatively you can try triggered updates on the links connecting to your DSL routers. This will send updates only when there is a change and keep the periodic updates suppressed.

Router(config-if)#ip rip triggered

Please do update what is the effect.

HTH-Cheers,

Swaroop