I've been in a similar situation testing a Bushnell camera trying to get photos of sika. I was of the view the deer were largely nocturnal. So far I haven't got a single photo of them at night but have managed quite a few of them during the day! It must be said that I'm putting the camera in cover and clearly they hold up in cover during the day so in the end it is to be expected that I'm seeing them in cover in daylight.
Aiming of the camera is very important. On the first 3 week stint my camera was out I didn't get a single photo of a deer and it was suggested that the sensor was aimed slightly too high - I've found that aiming in the vertical plane is crucial and aim too high or low and there will be no pictures on the card even on well used deer tracks with fresh slots on them when I go to pick the camera up. Maybe you are aimed too low and just detecting the foxes?
Another thing, and this might be the key to your problem, is that I think sika are clever enough and nervous enough to avoid the camera for a few days after it is put in place. I've found that if I leave it out for a week there are never any photos at all on it for the first few days and then the numbers start to pick up. Recently I left it in one location for exactly a week and had 102 photos, 99 of them with deer in them, but none were taken until about 4 or 5 days after the camera was in place.
My advice is to take a digital camera and some door wedges with you next time you position the camera. Set it up and then do some walk past tests, take the card out and view the results in your digital camera. Make sure that the centre of the image is right bang where you want to be detecting the deer - if it is a little high for example it might be shooting over the backs of the sika. Use the door wedges to adjust the camera so it is pointing to the right place. My experience is that your aiming needs to be much more accurate than you might imagine.
For example in the photo below I suspect it was the top of my head that triggered the camera, this camera didn't detect a single deer in 3 weeks because they were all passing below the sensor:
On the other hand this position took 99 pictures of deer in a couple of days and you can see the centre of the frame is aimed at deer height right across the open area:
Give this stuff a try before you blame the camera, I was doing the same thing until I realised how important accurate aiming was. Also, the Bushnell I have (Model number ending 437C) is available for about £112 in the USA. Fast trigger, good sensor, good quality photos, good flash range at reasonable money.