I’m knitting a baby hat that suggests a size 5 needle to get a gauge of 18 stitches for 4 inches. Since I’m a tight knitter, I usually up the needle size by one and it generally is fine. This time, I tried a size 6 needle and got something ridiculous like 30 stitches for 4 inches. I’m guessing that it may be that I’m using a thinner yarn, 142 yards per 50 grams. (Cashsoft DK) The pattern suggests 108 yds for 50 grams. (Tahki Cotton Classic) Any suggestions? Thanks a bunch!
It’s normal that you need more stitches with thinner yarn to get the gauge than with thicker yarn.
Your yarn is a little thinner than the one suggested, so that’s normal. If you don’t want 30 stitches instead of 18, you could switch to another yarn (less thin)
You can also try knitting with 2 strands together to mimic a thicker yarn.
or can you alter the pattern to work with the guage you have? what is the pattern, maybe you could simply cast on more stitches than the pattern calls for.
What is the suggested gauge for the yarn you are using? I usually don’t try to push the yarn more than a couple of stitches up or down if I want to keep a “normal” looking fabric. If the suggested gauge is much different from the one suggested for the hat yarn you will probably need to work the hat to a different gauge as Tarentella suggested. Or maybe a double strand will work as WildMountainHoney suggested. Often I find that although one strand won’t work, neither will 2. 
Two strands makes it quite a bit bigger and often gets too large.
Since one of you asked what the pattern is, I will try to condense it here. Then maybe one of you knowledgeable knitters can tell me if this works or "go have fun and buy different yarn!"
It has two parts: the brim and then the main part of bobbles.
CO 168 (for a 3-6 month baby) and decrease down to about 14 rows and bind off. (Row 2: *K5, k2tog, etc Row 3: *K4, k2tog, etc. Row 4: *K3, k2tog, etc. Bind off.
For the top part of the hat (the cute bobble pattern), pickup and knit 72 stitches evenly spaced along the purl ridge of the ribbed brim.
Knit the bobble pattern increase which is row 1 purl; row 2 p2, bbl inc to end of row; row 3 purl; row 4 p2, bbl dec to end of row; through row 8. You basically knit the bobble increase pattern for 3 rows, knit a bobble decrease for one row, go back to the bobble increase pattern and knit 3 rows and back to the decrease row.The total is 21 rows. (The bobble increase is p1, yo,p1, yo, p1 in same stitch).
Any further help is truly appreciated!