Share investment should be viewed as one of medium to long term duration if it is to yield a good increase in capital. It may often be tempting to sell shares if they reach a price considerably higher than the purchase price. You would do well not to base your decision to sell on the difference between the purchase price and current market prices.
When reviewing your portfolio the relative value of each share should be evaluated in light of current market conditions. If it is doing well, it will no doubt realise an attractive profit if sold, but would be better held as a solid maturing investment with attractive dividend potential By selling with the sole purpose of realising a profit within a year of purchase, one may be liable for capital gains tax.
Where a share price drops and remains at a low level for a while, consider your shareholding in this company. Study its records again.
Buying more of the same share when it is at a low price, with the idea of reducing the average price of the total number of shares may be unwise. You should rather find out why the share price has fallen. If the share is no longer a good medium for investment, improve your position, consider selling and selecting another in its place. You do not want to hold any shares when their value and marketability is low