CDO

Dear cdo users

Im using cdo to regrid a dataset (attached) but I am having some problems.
Im using:
cdo sellonlatbox,10.12,42.68,-36.96,-9.68 -remapbil,griddescription.txt input.nc output.nc
and I get:
cdo(2) remapbil: 5%cdo(2) remapbil (Warning): Bilinear interpolation failed for some grid points - use a distance-weighted average instead!

and it makes some of the grid points to go missing.
Can someone help me to understand why is the bilinear interpolation failing?

Climate Data Operators version 1.6.0 (http://code.zmaw.de/projects/cdo)
Compiler: gcc -std=gnu99 -fopenmp
version: gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
with: PTHREADS OpenMP Z
Compiled: by izidinep on pickle (x86_64-unknown-linux-gnu) May 14 2013 09:48:33
CDI library version : 1.6.0 of May 14 2013 09:48:28
CGRIBEX library version : 1.6.0 of Mar 5 2013 11:10:25
netCDF library version : 4.1.3 of May 1 2012 07:57:49 $
SERVICE library version : 1.3.1 of May 14 2013 09:48:23
EXTRA library version : 1.3.1 of May 14 2013 09:48:20
IEG library version : 1.3.1 of May 14 2013 09:48:21
FILE library version : 1.8.1 of May 14 2013 09:48:20

and this is the native grid
gridtype = curvilinear
gridsize = 38994
xname = lon
xlongname = longitude
xunits = degrees_east
yname = lat
ylongname = latitude
yunits = degrees_north
xsize = 194
ysize = 201
xvals = -24.64 ....

thanks in advance
izidine