Until you can get an exposure that gives you a "reasonable print" it will be difficult to judge how thin the neg is and thin negs can be due to underexposure and under development or a bit of both.
Is there any sections of the neg that looks very dark grey or black? If you can see practically see through almost any section of the neg then it is seriously thin. If so are they all like this? If not try to print from a "meatier" neg to see what this does to exposure. If exposure becomes close to say nearly double figures in seconds at about f11 then it would seem that thin negs are the issue rather than wrong bulb wattage
Very thin negs can be printed successfully but can be a pain and need to be avoided in the future. By and large following developer manufacturers' times avoid underdevelopment. Under exposure means that your metering and/or your film speed is wrong.
If your negs are really thin then with the next film I'd be inclined to either do a personal film speed and development test or if this seems a bit too complicated then expose each neg at meter setting initially then at 3/4 quarter speed and just over half speed so if the film is 400 then expose at that then 320 and 250 or try 320 and 200.
Develop as per instructions time for box speed. Finally see which negs produce the best prints in terms of shadow detail. If this is a neg exposed at half speed it is likely that the highlights might be blown i.e. almost pure white. At that point you have your correct speed and can then reduce development to avoid over development.
However this is probably getting ahead of myself. First step is to check what wattage the bulb should be and then what yours is.