Increased shared_buffer setting = lower hit ratio ?

All;

We have a large db server with 128GB of ram running complex functions.

with the server set to have the following we were seeing a somewhat low
hit ratio and lots of temp buffers

shared_buffers = 18GB
work_mem = 75MB
effective_cache_size = 105GB
checkpoint_segments = 128

when we increased the values to these not only did the hit ratio drop
but query times are now longer as well:

shared_buffers = 28GB
work_mem = 150MB
effective_cache_size = 105GB
checkpoint_segments = 256

This does not seem to make sense to me, anyone have any thoughts on why
more memory resources would cause worse performance?

Thanks in advance