Rotary valves can be a bit of a mystery for horn players and musical instrument repairmen. Three topics you should consider.
Strings too tight. I’ve seen this more often than I would like. I think that at the shop the repair person thinks that the strings should be really tight and it will keep things quiet or something. Reality is that if the strings are way tight it holds the valve over to one side of the bearing instead of letting it spin freely. Your valves may catch or not even go all the way up or down. You don’t want them to be loose, but don’t have your strings very tight, find a happy medium where the valves spin easily.
Thin oil. I think part of the problem is that people are very influenced by marketing. If you have rotary valves you have to use rotary valve oil, right? But reality is if you have an expensive horn with high tolerance valves, oil that is too thick just slows everything down. Speaking generally use thin valve oil and use it often, unless there is noise in the bearings or linkages and then use as thick as you need.
Alignment. It seems real basic but it is important that the marks on the top of the valves actually lines up correctly. If they don’t, your horn won’t play optimally. Remember that thousandths of an inch matter in horn construction, and if any of your valves are rotating say 1/16” further than those marks, that is just not a good thing at all. If not rotating far enough, take a sharp X-Acto knife and trim the bumper. If rotating too far, replace the bumper.
I’ve seen horns come back from shops with poor alignment. I think what they did was just use the type of bumper stock they have and they did not modify it to get the alignment right. Those same shops may offer “scope alignment” of trumpet valves – they would not think of sending out a trumpet with valves 1/16” out of correct, but horn is another story.
In the case of all of these things, these are not things to be afraid of, take control and make your valves work better.