David your last comment hit the nail on the head:
but perhaps still not eating enough.
The equation is simple. Eat 500kCal/day more than you burn and you will gain a pound a week. Since I started working out again I’ve gained exactly a pound a week for a total of 7 pounds. I will work out for another 3 weeks gaining 3 more pounds before taking a one week break at which point I will start again for 10 weeks and 10 more pounds at which point I’ll evaluate how I want to continue.
I use to think of myself as a “hardgainer.” After I started powerlifing I realized that is an often overused excuse. “Hardgainers” are almost always over/under-trainers and/or under-eaters and it’s usually the latter, since 3500kCal a week equals a pound. Training will just help more of that be muscle than would have otherwise. If you are not gaining weight you aren’t eating enough plain and simple.
When I started power lifting I gained 40lbs from 125 to 165 in one year and never had my body fat go over 12%.
In order to get enough calories I eat 8 meals a day about 3 hours apart (including whenever I wake up during the night). 4 of those are shakes one every 6 hours. In between are normal meals except for workout time when it’s a pre and post workout shake. If not for the shakes it would be hard to eat the 2500-3000kCal/day I need to stay 500kCal above maintenance. The 2500-3000kCal range takes into account whether I work and or work out on a given day which both up my requirements which I adjust for accordingly.
One advantage of doing the 4 really 5 shakes if you count pre/post workout is that gives me a set number of kCal/day, about 1700 so I only have to count the extra I need to make up the difference.
It can take some time to dial in exactly how much you need to gain a set amount so when in doubt eat more and monitor your weight gain adjusting down or up as needed. If you don’t already have one Amazon sells some inexpensive skin calipers under $10 so you can monitor your body fat.
As for workout what do you do? Can you squat 1.5 x your body weight or better? I ask because you may not be doing a good workout for bulking. Just eating a lot w/o the right exercises means more of what you gain will be fat if you start eating more.