never, ever increments

It's a strange quirk of C. The "++" operator is the post-increment operator. It means to use the value of the variable in an expression, and then increment it. In this case, the expression is (sbuf0Tail % BUFFER_SZ). Let's say sbuf0Tail is 0. The expression value is then (0 % 64), or 0. Remember that. Now increment sub0Tail to 1. Now finish the assignment statement: assign the remembered 0 to sbuf0Tail.

You either want to pre-increment, or just write two separate statements to avoid confusion. I'd recommend the latter. Advanced students can study the rules about "sequence points" for extra credit.

sBuf0Tail++;
sBuf0Tail = sBuf0Tail % BUFFER_SZ;

Incidentally, many (but not all) compilers are smart enough to strength-reduce that division to an & when the RHS is a power of 2. But division is expensive, and I don't think I've ever seen a compiler that avoids division when the RHS is not a power of two. I usually code this idiom as

sBuf0Tail++;
if (sBuf0Tail >= BUFFER_SZ) sBuf0Tail = 0;

which on most architectures is almost as cheap as the AND, and much faster than division when you can't just AND.

Maybe
sBuf0tail = ((++sBuf0tail) % BUFFER_SZ);
will work.
++X is not the same as X++ in all cases.

Look up pre / Post increment in your C book.

"Advanced students can study the rules about 'sequence points' for extra credit."

You could start by just putting "sequence point" into the search function on this forum - it's been discussed a number of times before...

Rather than just one reference, several from http://www.eskimo.com/~scs/C-faq/s3.html seem to apply here, so I'll link the whole lot.

I know referencing FAQs is the lazy man's approach, so call me lazy.

"Maybe sBuf0tail = ((++sBuf0tail) % BUFFER_SZ);
will work.
++X is not the same as X++ in all cases."

It's undefined behaviour either way.