This is by design. Restaurants will sometimes only give OpenTable a few slots a night as OT often takes a good chunk of margin from the restauranteurs:
'The access fees can be substantial, particularly for restaurants operating on thin margins. One independent study estimates that OpenTable’s fees (comprised of startup fees, fixed monthly fees, and per-person reservation fees) translate to a cost of roughly $10.40 for each “incremental” 4-top booked through OpenTable.com. To put that in perspective, consider that the average profit margin, before taxes, for a U.S. restaurant is roughly 5%. This means that a table of 4 spending $200 on dinner would generate a $10 profit. In this example, all of that profit would then go to OpenTable fees for having delivered the reservation, leaving the restaurant with nothing other than the hope that that customer would come back (and hopefully book by telephone the next time).'
'The access fees can be substantial, particularly for restaurants operating on thin margins. One independent study estimates that OpenTable’s fees (comprised of startup fees, fixed monthly fees, and per-person reservation fees) translate to a cost of roughly $10.40 for each “incremental” 4-top booked through OpenTable.com. To put that in perspective, consider that the average profit margin, before taxes, for a U.S. restaurant is roughly 5%. This means that a table of 4 spending $200 on dinner would generate a $10 profit. In this example, all of that profit would then go to OpenTable fees for having delivered the reservation, leaving the restaurant with nothing other than the hope that that customer would come back (and hopefully book by telephone the next time).'
From http://insidescoopsf.sfgate.com/blog/2010/10/18/is-opentable...