Cart Recently Stopped Working

Hi all hope someone has some ideas on this my cart has decided to not let a certain product from being added to the cart, you click add to basket but when you check the basket nothing is added. I have tried disabling all my plugins but the problem is still there. The item in question is Reishi capsules. What can I do to fix this? and what would of made this error start to occur??

vitalherbs.co.uk this is my website , hope someone can help im losing customers and my hair thanks Neal
my theme is wpstore