Posting for some suggestions please 
This is DC4 and is now approx 4 months. Sleeps through the night most nights except since the heat last week but will NOT nap in the cot during the day. It's probably coincided with being a bit more alert during the day, say around 8 weeks ago. But it's getting to the stage where I think it's probably affecting night time sleep. As soon as I put her down in the cot she really screams, sounds super annoyed! I calm her and try again, same result, leave the room, go back in after a minute and repeat. Nothing works. We have someone working for us on the floor above and I don't like to have her crying as it disturbs them.
I've discovered in the last few days that she will sleep very happily in the pram downstairs and we have in fact got two long naps and one short one before dinner consistently last week this way.
However, I'm not sure I'd like the pram as a sleep place for too long. I'd like her to have the option of the cot and be happy there. I have another DC who still needs a nap so I tend to be at home for that one which means the baby can sleep in her cot for at least one nap.