It’s probably not tight enough. You should try it with other characters with 3 frame jabs and see if you are trading at times. If so, you are a frame too late with the frame trap (well two frames too late since it should be a 1 frame frame trap).
Krazysh0t hit on the right answer. First, it’s important to understand that there are two fundamental classes of characters in this game for frame-trap purposes:
Characters whose fastest normal is a 3 frame start-up (Chun-Li, Ryu, Cammy, Necalli, Vega, Laura, R. Mika, Karin, and Rashid)
Characters whose fastest normal is a 4 frame start-up (Ken*, Zangief, Nash, M. Bison, Birdie, F.A.N.G., and Dhalsim)
Ken’s LP Shoryu and LK Tatsu are 3f, but his normals are all 4f or slower
So, some traps will work vs. the 4f characters, and not the 3f ones. This is probably why you’re finding it more difficult to trap Chun-Li than M. Bison - Bison’s c.lp takes 1f longer to come out.
For example, with Cammy, s.lk, c.mp or c.mp, c.mp are both frame traps vs. 4 frame characters, because s.lk and c.mp are +1 on block, and c.mp is a 5f medium attack. After blocking a s.lk/c.mp, your c.mp and their c.lp will both become active on the exact same frame, but c.mp will win because in SFV the higher strength button always wins in a direct trade. However, those same strings will not work against the 3f characters, because their 3f attack will arrive 1f before your c.mp and counter hit you.
Krazysh0t is right that you are probably messing up the input a little, and not hitting the second button in the trap soon enough. In fact, you should even be able to trap 3f characters with s.mp (blocked), c.mp. Since you are +2 after blocked s.mp, your c.mp (5f startup) will arrive at the same time as their 3f button, and will win because, again, medium attacks beat simultaneous light attacks. No character in the game has a 3f medium attack, and I can only think of two that have 4f mediums (Ken and Cammy both have 4f b+mp attacks).