Today I came across the same problem, but finally found out what's going on with HMCS logic. So here is a quotation from ED's "DCS 2.8.0.32235.1 Open Beta" changelog that should explain it:
Updated ACM BORE HMCS logic. Hold TMS Forward to slave radar to HMCS line-of-sight and display the ellipse. When TMS Forward is released, the radar will attempt to lock the nearest contact within the ellipse out to 10 nm.
-so you have to hold TMS forward when aiming and release it when aimed on your target, than radar will attempt to lock a target
I don't understand why it's like that, to me it seems very impractical and it doesn't meke sense to meke it like this IRL. But hey, it is what it is. If it's realistic now, than it's very impractical but we have to deal with it and if not then hope ED will fix it.