Rule n°1: if it ain't working out don't insist. In this case if your third and fourth sets won't go in without brute force and ignorance it's time to stop, take it apart, check the lengths and positions of everything and start again - before you have put a terminal kink in the rim. I am a skinflint who regularly builds wheels (only for myself) with mismatched components (as in salvaged scrap!)and this happens. Above all never insist!
Possible areas of error: 1 the measures of rim, spokes or hub are quite simply wrong. It happens. Measure everything yourself to be sure. Look up in wheelbuilding manuals or on the net how to measure spokes (don't ask me, I never know). Don't trust the word of anyone, particularly not internet databases or salesmen.
2: You don't say what the hub is. Some dynohubs (I think all disc brake dyno hubs) are not symetrical. Have you got the hub the right way round?
3: The holes in the rim are offset from the centreline. Have you got the rim the right way round? (I have always built right side of the rim for right side of the hub - I assume this is correct, it works for me. If not I have been wrong for half a century. I have often wondered if the wheel would be stronger with right going to left to increase the angle a bit but I don't think I am going to try! Anyway the spoke lengths would be wrong
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4: I get all the spokes in before putting any tension on anything. Just three or four turns on the nipple to make sure it's threaded. Done this way all the spokes should be well loose when put in place. If not something is wrong!
5: Everyone has his own way of lacing up a wheel. My way is to start with the narrow side (least dished side, as in drive side on a rear, disc side on a front disc) putting the spokes in from the outside. Then for the same flange I put in two spokes inside going out and crossed correctly. These two spokes will give me the flange hole for the spokes on the other flange (either leading or trailing to give the right direction at the valve hole). The first set of spokes on the second flange go from the outside in (which is why there are only two spokes on the opposite flange, to make putting them in position easier). Finally it's just a question of finishing off the remaining flanges on which the spokes go inside out - which makes life easier. If you get to the last set and they are too short then something is wrong somewhere; time to stop and start again, or give it a rest and come back two days later when all will be fine.
6: don't forget that the holes on the two flanges are not face to face. One side will lead the other (by half the distance between two holes usually. There are some strange radial wheels which don't follow this exactly). Pass a spoke through one hole and if it is parallel to the axle it should hit the other flange between two holes. You need to have the spokes leaving the hub in the same direction from the twp flanges with the flange holes correct, leading or trailing, otherwise you're doomed!
