Many dogs are fully house trained by 6 months, but some aren't, so she may just be one of those. If she was mine I'd feed her evening meal later, at 6.00 or 7.00pm, which might increase her chances of being able to hold on all night while her evening meal is being digested. Feeding early can mean they're hungry and wakeful during the night, which probably increases the likelihood of needing to poo then.