My granddaughter is 18 months old and my daughter tells me that occasionally she won't eat her dinner ( at tea time, sandwiches at lunch) These are meals she's been very happy to eat on other days. So my daughter just leaves it and doesn't press her but encourages her but this doesn't work if she's in the mood to not eat. When this happens she might be given a plain biscuit or cracker with her evening milk (at 6.30) The next day Sophie usually is eating as good as gold.
Hope this helps - kids are funny!