"We walk with you every day."
Translation:Siúlaimid leat gach lá.
You're correct. leatsa is the emphatic.
Siúlann siad leí gach lá ach Siúlann muidne leatsa gach lá
leat is referring to one person, libh is referring to two or more people.
"I walk with you" should accept both Siúlim leat and Siúlim libh.
I think some recent change at Duolingo has broken something, and valid alternative answers sometimes being rejected, but until people start taking screenshots that demonstrate these issues and flooding the Troubleshooting discussion with them, I don't know if the Duolingo engineers will even notice that there's a problem - they certainly won't see any of the complaints in sentence discussions.