Help needed with seedlings???

Hi there, Just joined your forum today this is my first post. Moved up to brisbane from melbourne recently and have been having problems with my plants since then, my grow setup is simple, small cupboard, inside grow, 250w hps, growing in soil and using bottled water
this has worked well in the past but since moving my seedlings get to about 2 inchs / 50mm high stop growing, slowly go yellow and die. Have tried different soils, different water with no luck, temp inside the box is about 22-28 degrees c. What am I overlooking?
Any help would be great

Early signs of yellowing could indicate overwatering because it locks out the nitrogen, your plants are looking healthy I wouldn't worry to much wait till about the 3rd node then start feedling nutrients , I have never grown in soil but I read ph should be around 6.5 if it is reaching over 7.0+ range it might cause your seedlings to slow in growth and stop the plant from taking in some nute's. I am new also so I hope you get some pros here to shed some light also temp/humidity?

Thanks for your advice, I try and let the soil dry between watering but maybe i am still watering to much, the current plants have not started going yellow yet but have definitely slowed down in the last couple of days, just want to get on top of this problem before i lose this one as well.
Temp ranges from 22c to 28c humidity 40-60%

Those numbers seem fine for this stage that's about where mine are just try to keep them under that anything higher than 27c will definitely encourage rot with a lot of water. Soil temp should always be cooler then room. losing seedlings suck trust me I know but try to learn from those and apply it too this one. I think it is better to start seedlings in a smaller pot have you considered rapid rooters or peat pellets its a good way to tell how moist they should be and retain a lot of air/water radio. The soil in your pic doesn't seem to be overwatered seems about right, how old are they??

Plants are about a week and a half old but have not grow much at all in the last few days