Error gstat function krigeST

Dear all,

I am working with an hourly dataset of air temperature, recorded at ~200 
stations over a relatively small area. I chose a space-time variogram 
(e.g. sum-metric) to fit my data and am now trying to make predictions 
over my same stations in order to fill NA (missing value) gaps. When 
using the krigeST() function over daily aggregated data everything seems 
to go smooth but when I use it at the original hourly resolution I 
always get the following error:

Error in chol.default(A)
the leading minor of order 68 is not positive definite

I googled it and found that it is related to a matrix not being 
completely positive-definite. However, I am not sure why this happens 
and was wondering if any of you know a way of fixing this (a workaround 
to avoid it).

Thanks!
Francesco