Prads,
can you ascertain that you really have a 10MHz signal coming into your arb? You have the 5142 there (digitizer) - you could just hook up the clock output of 5610 to channel input on the digitizer, and make sure that you really have the 10MHz as per specification.
I suspect a bad cable or a bad external connection - but the only way to make sure is to test the cable with the expected signal. Make sure that you use the same cable when testing the clock (this may be tricky as the scope's input is BNC and the arb's is SMB, but if you have an appropriate adapter that could help.
best regards,