If you're still having trouble gaining weight, a good trick to add to your hardgainer workout is high volume training for a few weeks. Now, one very important point here is that I am NOT suggesting you do a million exercises for 2 hours a day 6 days a week.
This is the most detrimental thing you can do as hardgainer. You will see much better results training hard with just a few basic compound exercises a couple days a week the main focus on increasing strength.
However, if you have reached a level in your hardgainer workout where you have increased your strength significantly but still lack mass, try a little high volume training.
Here's an example of an abbreviated hardgainer workout routine that I would normally recommend for hardgainers to first increase their strength:
1. Squats: 5x5
2. Bench Press: 5x5
3. Weighted Pull-Ups: 5x5
4. Military Press: 2x12
5. Romanian Deadlifts: 5x5
This is a basic version of the 5x5 system that has been proven for decades to increase strength like no other.
Now, to add some high volume training in your hardgainer workout, let's use the bench press for example:
After two progressively heavier warm up sets for 5 reps, you do your last 3 sets of 5 with the near maximum weight you can handle for 5 reps. Immediately after this, you strip the weight to an amount that you can do 10 reps for in a relatively fast fashion.
After 30-45 seconds rest, do another 10 reps. Continue with this, while stripping the weight as necessary to maintain 10 clean reps per set, until you do a total of 8 to 10 sets. By the last few sets you probably will only be able to just do the bar, but that's OK. You've already done your strength training with your first 5x5, now the point is to really get the blood pumping in your muscles.
So, the entire process for just the bench press would look like this:
Bench Press: 5x5 strength training; then 10x10 w/30-45 second rest between sets
When doing high volume training like this, it is a good idea to abbreviate your program even further, such as just picking three exercises a day.
Obviously you won't be able to do strength training for the military press on the same day as the bench press with this, since your triceps will absolutely be fried.
Try this high volume training in your hardgainer workout for the most basic compound exercises for a while if you are hitting a plateau in your gains lately, and you'll more then likely add a few extra pounds within a matter of weeks.