If your engine is a little bit older (meaning you can't remember when the last rebuild was), the lifters might be bleeding off pressure while you're adjusting your valves. With hydraulic lifters it's important to have
oil in them to get an accurate setting. If you keep adjusting them with your "static" method, but they are still not right, bleeding might be the reason. In that case you might want to: 1) Adjust them faster;
2) Replace your lifters; or 3) Adjust them with the engine running (yes that is messy). You know with today's quality valve train components, solid lifter adjustment is not what it used to be when we were kids (meaning having to adjust your valves every other weekend). Most FEs with solid lifters in Cobras now-a-days only requiring checking the valves once or twice a year (because of the typical lesser amount of use) and rarely require actual adjustments. That's assuming you have good quality stuff under the valve covers.