The question - why did the CME Group’s 5 margin increases in 8 days not impact those short silver futures the way it impacted those long silver futures?

The answer is that JPMorgue acting as a broker dealer did not put up additional cash margin for their short positions. In fact, JPM placed the increased long margin paid in into their JPM bank accounts, not in CME accounts. When the JPM broker dealer has an equal number of long and short positions JPM is not required to deliver the long and short cash margins to CME but hold them their self. Any net cash requirements owed by JPM is covered by a "Bank Letter of Credit" on file at CME. The bank letter of credit is in effect a check payable on demand (trust us we are good for the debt, just hold that piece of paper.)

A long discussion of these shenanigans and a list of banks qualified to issue letters of credit are included in the following links:

http://www.marketskeptics.com/2010/04/net-settlement-and-cmes-missing.html

http://www.marketskeptics.com/2010/04/shortfalls-of-clearinghouses-guarantees.html

http://www.marketskeptics.com/2010/03/standby-letters-of-credit-and-clearing.html

My starting point for finding the above links was the following zerohedge article:

http://www.zerohedge.com/forum/why-jp-morgan-short-silver-and-gold

The concluding paragraph from zerohedge:

If the CFTC changed the rule to require that all parties post margin on their own position (therefore this does not affect clearing members that do not trade for their own account) AND mark these to market I doubt very much that we would see the shenanigans or huge positions that we currently see. This also goes a long way to preserving the integrity of the market as genuine buyers and sellers would know that their positions would not be at risk. If you bought at X and sold at X+1 and the market traded up to X+10 and then someone called you to tell you that the person you thought you bought at X from was out of business, you would actually be short at X+1 with the market 9 points against you. Expand this thinking to large physical trades and you begin to see the type of exposure the market faces when the clearing houses take their own positions with no risk or even reportability to their own top management. Most companies do not pick up on rogue trading exposure until margin requirements trigger internal cash calls. With the "net" CME system there are never going to be cash calls and there is no mechanism to catch rogue or excessive trading.

"If you don't hold it, you don't own it." The only way to invest in precious metals is buy, take delivery, and hold the metal. Any of the paper derivatives will ultimately result in losses.