Touch sensative door handle locking

i recently purchased an 09 astra twin top and it comes with keyless entry. After reading the manual i descovered you can lock the doors by just touching the 2 small lines on the either passenger or driver door handles. I love this option and its great when it works. The problem is it seems to only work 5/10 times. Sometimes it works perfect sometimes not at all. Sometimes will work on driver but not passenger sometimes on both. Im not sure if its external temperiture or air moisture or what....

Touch sensative door handle locking