My new I-20 and also I-20 SS have problems rising unless I grab the rising part and pull up, which is totally impractical. The problem is not for all of the Hunter I-20 and I-20 SS I replaced last week. 7 others (combined number of I-20 and I-20 SS) that I replaced at the same time are working fine. I read in a forum that someone was recommending to remove and clean the top assembly to get sands/soil out of it with water. Mine are new but cleaned them anyway just to make sure nothing foreign got stuck when I first installed them last week. Naturally, I suspected water pressure but the area where they are installed, namely Scarsdale, NY, is known for high water pressure among cities/towns and villages in Westchester County (NY). I am considering to change them to yet another Hunter sprinkler head model I-20 Pro. I know there is no guarantee but am just hoping it may solve my problem. Learningat80, Fingers Crossed!
June 22, 2017 14:00 EDT
Thank you for the suggestion, to try changing the head with known good ones.
I will do and come back in a day or so with my report.
In the meantime, this sprinkler system is 20 year old and has been working fine until I recently replaced some of the heads with new ones. The systems originally had Toro heads, which I replaced with Hunter PGP heads one by one during the past 20 years whenever I noticed problems with a certain head. Those heads I have problems of Rising are at the same location where older PGP and I-20 used to work fine. I wonder whether I-20 (I-20 SS) heads are equipped with springs stronger than those used in PGP, requiring more water pressure to allow the head to rise?

Please let me know where I can write in response to yours. Without the knowledge, I wrote as an edit to my original.
June 23, 2017 9:10am E.D.T.
So, not knowing where/how to write my follow-up report, here is what I found out:
Replaced at 8:30am a known good riser with a known bad riser(that does not rise without my pulling it up).
Now, they both work fine without my help. It so happened that those I swapped were both of I-20 SS model that were at locations easy for me to swap. My sprinkler systems is usually set to start at 4:30am but when I was checking on the operation of these risers were often during the day when water pressure might have been low. The village engineer confirmed that a typical water pressure for this area is 66 PSI with minimum of 30 PSI as set forth by the village code but it never goes that low. I am wondering where I can find the water pressure in Hunter's I-20 sprinkler head specs. If you know, please let me know.
Meanwhile, I correct what I referred to as "I-20 Pro". I should have said "I-20 Ultra".