I've had really good trips off half eighths and at times I'v felt nothing off of 2 grams...just depends on the batch and obviously set and setting. Maybe them being first timers has something to do with it. The first time I popped a tab of acid I felt nothing, then a week later I popped my remaining tab from the same batch and tripped balls. I've even seen people convinced that nothing is happening but they spent all night staring at the wall with a creepy grin. I never trip on an empty stomach. From personal experience I'd say 1.5g of cubensis is a really small dose even for a first timer, unless they're exceptionally strong. Although they should have felt at least a body buzz or that confusing "something is happening but I can't quite put my finger on it" low dose high. Just up the dose next time. 2g or 2.5g, heck even an 8th. I've dose a lot of first timers either with 3.5g or 2 blotters and haven't had a freak out ever. Perfect set and setting is where its at.